improve error message on invalid formatting of shell options

Problem: When submitting a job through the command line, a shell
option `-o KEY[=VAL]` is not required to have a value. If one is
not provided, then `attributes.system.shell.KEY` is set to 1. If a
user later provides a sub-key of this shell option, such as `-o
KEY.foo=bar`, the error message returned by Python is not
particularly helpful.

Wrap `set_treedict()` in a try/except where user input might
provide an invalid key and raise a useful error message.
Fixes #6678

src/bindings/python/flux/cli/base.py

@@ -422,7 +422,10 @@ def update_keyval(self, keyval):
             value = json.loads(value)
         except json.decoder.JSONDecodeError:
             value = str(value)
-        set_treedict(self.config, key, value)
+        try:
+            set_treedict(self.config, key, value)
+        except TypeError as e:
+            raise TypeError(f"failed to set {key} to {value}: {e}")
         return self
 
     def update_file(self, path, extension=".toml"):

src/bindings/python/flux/job/Jobspec.py

@@ -564,7 +564,10 @@ def setattr(self, key, val):
             if not key.startswith(("user.", "system.")):
                 key = "system." + key
             key = "attributes." + key
-        set_treedict(self.jobspec, key, val)
+        try:
+            set_treedict(self.jobspec, key, val)
+        except TypeError as e:
+            raise TypeError(f"failed to set {key} to {val}: {e}")
 
     def setattr_shell_option(self, key, val):
         """