Catch KeyError as a failure of multithreading as well. (#710)

tests/test_synced_collections/synced_collection_test.py

@@ -481,7 +481,7 @@ def set_value(sd):
         try:
             with ThreadPoolExecutor(max_workers=num_threads) as executor:
                 list(executor.map(set_value, [synced_collection] * num_threads * 10))
-        except (RuntimeError, JSONDecodeError):
+        except (RuntimeError, JSONDecodeError, KeyError):
             # This line may raise an exception, or it may successfully complete
             # but not modify all the expected data. If it raises an exception,
             # then the underlying data is likely to be invalid, so we must