Make failure to lookup Slurm reason description non-fatal

Alas there are a number of omissions in SlurmJobExecutor._REASONS_MAP.
Specifically the one we encountered was OutOfMemory, but there are
quite a few others missing.  Trying to add them all seems not
worthwhile.  Instead reason lookup has been made robust.

Author: Simon Guest

src/psij/executors/batch/slurm.py

@@ -180,8 +180,9 @@ def _get_state(self, state: str) -> JobState:
         return SlurmJobExecutor._STATE_MAP[state]
 
     def _get_message(self, reason: str) -> str:
-        assert reason in SlurmJobExecutor._REASONS_MAP
-        return SlurmJobExecutor._REASONS_MAP[reason]
+        return SlurmJobExecutor._REASONS_MAP.get(
+            reason, f"{reason} - no description available"
+        )
 
     def job_id_from_submit_output(self, out: str) -> str:
         """See :meth:`~.BatchSchedulerExecutor.job_id_from_submit_output`."""