Avoid spurious backtrace / log message on critical error (#291)

## Problem

On a critical error (e.g. existing index and --overwrite not specified),
VSB prints a large amount of spurious / unnecessary output - e.g:

```
vsb --database=pinecone --pinecone_api_key=XXX
    --workload=synthetic-proportional \
    --synthetic_records=1000 --synthetic_requests=100 \
    --synthetic_insert_ratio=0.3 --synthetic_query_ratio=0.5 \
    --synthetic_delete_ratio=0.1 --synthetic_update_ratio=0.1 \
    --synthetic_dimensions=768 --synthetic_query_distribution=zipfian \
    --synthetic_metadata=id:10n --synthetic_metadata=tags:5s10l \
    --pinecone_index_name=daver-synthetic-test
2025-01-10T14:44:11 INFO     Vector Search Bench: Starting experiment with backend='pinecone', workload='synthetic-proportional', users=1, requests_per_sec=unlimited
...
2025-01-10T14:44:12 CRITICAL PineconeDB: Index 'daver-synthetic-test' already exists - cowardly refusing to overwrite existing data. Specify --overwrite to delete it, or specify --skip-populate to skip population phase.
Traceback (most recent call last):
  File "/Users/dave/repos/pinecone-io/VSB/vsb/users.py", line 64, in setup
    self.database.initialize_population()
  File "/Users/dave/repos/pinecone-io/VSB/vsb/databases/pinecone/pinecone.py", line 155, in initialize_population
    raise StopUser()
locust.exception.StopUser
Performing Setup phase                         0/? ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━  0:00:00   %
```

This causes the "CRITICAL" message to get somewhat lost.

## Solution

Modify the handling of critical errors to use an explicit DoneFailed
state in the state machine, which handles the shutdown without adding
backtraces, also also updating messages to be more applicable to a
failed run:

```
2025-01-10T14:47:20 INFO     Vector Search Bench: Starting experiment with backend='pinecone', workload='synthetic-proportional', users=1, requests_per_sec=unlimited
...
2025-01-10T14:47:22 CRITICAL PineconeDB: Index 'daver-synthetic-test' already exists - cowardly refusing to overwrite existing data. Specify --overwrite to delete it, or specify --skip-populate to skip population phase.
✘ Setup cancelled                              1/1 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:00 00:00 ~
```

## Type of Change

- [x] Bug fix (non-breaking change which fixes an issue)

vsb/users.py

@@ -74,6 +74,20 @@ def setup(self):
                     # Nothing more to do, but sleep briefly here to prevent
                     # us busy-looping in this state.
                     gevent.sleep(0.1)
+        except StopUser:
+            # Explicit stop, assume callee already logged the reason; exit with
+            # non-zero:
+            logger.debug("Stopping user due to StopUser exception")
+            self.environment.runner.send_message(
+                "update_progress",
+                {
+                    "user": 0,
+                    "phase": "setup",
+                    "error": "Stopping user due to StopUser exception",
+                },
+            )
+            self.state = self.State.Done
+
         except Exception as e:
             traceback.print_exception(e)
             locust.log.unhandled_greenlet_exception = True
@@ -375,6 +389,9 @@ class Phase(Enum):
         """Issue requests (queries) to the database and recording the results."""
         Done = auto()
         """Final phase when all Run Users have completed"""
+        DoneFailed = auto()
+        """Final phase when all a fatal error occurred and we want to stop down, 
+        signalling an unsuccessful run"""
 
     def __init__(self):
         super().__init__()
@@ -469,6 +486,10 @@ def tick(self):
                 return self.num_users, self.num_users, [RunUser]
             case LoadShape.Phase.Done:
                 return None
+            case LoadShape.Phase.DoneFailed:
+                # Set exit status to 2 to indicate a failure
+                locust.log.unhandled_greenlet_exception = True
+                return None
             case _:
                 raise ValueError(f"Invalid phase:{self.phase}")
 
@@ -486,9 +507,11 @@ def _transition_phase(self, new: Phase):
         ]
         if vsb.progress is not None:
             self._update_progress_bar(mark_completed=True)
-            vsb.progress.update(
-                self.progress_task_id, description=f"✔ {self.phase.name} complete"
-            )
+            if new == LoadShape.Phase.DoneFailed:
+                msg = f"✘ {self.phase.name} cancelled"
+            else:
+                msg = f"✔ {self.phase.name} complete"
+            vsb.progress.update(self.progress_task_id, description=msg)
             vsb.progress.stop()
             vsb.progress = None
         if hasattr(self.runner.environment, "workload_sequence"):
@@ -514,7 +537,10 @@ def _transition_phase(self, new: Phase):
         else:
             phase_display_name = self.phase.name
         if self.phase in tracked_phases:
-            metrics_tracker.record_phase_end(phase_display_name)
+            # Skip reporting phase end on a fatal error; it adds noise after
+            # the important fatal error message.
+            if new != LoadShape.Phase.DoneFailed:
+                metrics_tracker.record_phase_end(phase_display_name)
         self.phase = new
         if hasattr(self.runner.environment, "workload_sequence"):
             workload = self.runner.environment.workload_sequence[
@@ -539,9 +565,7 @@ def _transition_phase(self, new: Phase):
     def on_update_progress(self, msg, **kwargs):
         # Fired when VSBLoadShape (running on the master) receives an
         # "update_progress" message.
-        logger.debug(
-            f"VSBLoadShape.update_progress() - user:{msg.data['user']}, phase:{msg.data['phase']}"
-        )
+        logger.debug(f"VSBLoadShape.on_update_progress() - {msg.data}")
         match self.phase:
             case LoadShape.Phase.Setup:
                 assert msg.data["phase"] == "setup"
@@ -557,7 +581,11 @@ def on_update_progress(self, msg, **kwargs):
                     f"{self.request_count} - "
                     f"moving to TransitionFromSetup phase"
                 )
-                self._transition_phase(LoadShape.Phase.TransitionFromSetup)
+                if "error" in msg.data:
+                    # Setup signalled an error; cancel the benchmark
+                    self._transition_phase(LoadShape.Phase.DoneFailed)
+                else:
+                    self._transition_phase(LoadShape.Phase.TransitionFromSetup)
             case LoadShape.Phase.TransitionFromSetup:
                 logger.error(
                     f"VSBLoadShape.update_progress() - Unexpected progress update in "