Print exceptions raised during hardware sims

Previously exceptions raised during hardware sims would hang
because a channel was expecting data to be read or written,
causing deadlock. This write bogus data to unstick the simulation
while printing the exception so it can be debugged.

nengo_loihi/simulator.py

@@ -1,5 +1,6 @@
 from collections import OrderedDict
 import logging
+import traceback
 import warnings
 
 import numpy as np
@@ -485,7 +486,25 @@ def run_steps(self, steps):
             raise SimulatorClosed("Simulator cannot run because it is closed.")
         if self._run_steps is None:
             self._make_run_steps()
-        self._run_steps(steps)
+        try:
+            self._run_steps(steps)
+        except Exception:
+            if "loihi" in self.sims and self.sims["loihi"].use_snips:
+                # Need to write to board, otherwise it will wait indefinitely
+                h2c = self.sims["loihi"].nengo_io_h2c
+                c2h = self.sims["loihi"].nengo_io_c2h
+
+                print(traceback.format_exc())
+                print("\nAttempting to end simulation...")
+
+                for _ in range(steps):
+                    h2c.write(h2c.numElements, [0] * h2c.numElements)
+                    c2h.read(c2h.numElements)
+                self.sims["loihi"].wait_for_completion()
+                self.sims["loihi"].n2board.nxDriver.stopExecution()
+                self.sims["loihi"].n2board.nxDriver.stopDriver()
+            raise
+
         self._n_steps += steps
         logger.info("Finished running for %d steps", steps)
         self._probe()