Apply suggestions from code review of branch stitching-cancel

Co-authored-by: Richard Bowman <richard.bowman@cantab.net>

src/openflexure_microscope_server/things/smart_scan.py

@@ -319,7 +319,8 @@ class SmartScanThing(OFMThing):
             # Error must be raised so UI gives correct output
             raise e
         except lt.exceptions.InvocationCancelledError as e:
-            # InvocationCancelledError is caught for scanning but not for stitching
+            # InvocationCancelledError is caught in `_run_scan` if it happens
+            # while scanning, but it propagates if it occurs during stitching.
             # If raised we still need to save the scan log.
             if self._ongoing_scan is not None:
                 self._ongoing_scan.save_scan_log(self)