Add UNKNOWN status code to retryable Quantum Engine errors (#6565)

* Add UNKNOWN status code to retryable errors.

* Update test

* Revert unintended changes.

* More removals.

* Lint formatting

Author: wcourtney

cirq-google/cirq_google/engine/stream_manager.py

@@ -27,6 +27,7 @@
 RETRYABLE_GOOGLE_API_EXCEPTIONS = [
     google_exceptions.InternalServerError,
     google_exceptions.ServiceUnavailable,
+    google_exceptions.Unknown,  # 408 Timeouts sometimes show up as UNKNOWN.
 ]
 
 

cirq-google/cirq_google/engine/stream_manager_test.py

@@ -364,6 +364,7 @@ async def test():
         [
             google_exceptions.InternalServerError('server error'),
             google_exceptions.ServiceUnavailable('unavailable'),
+            google_exceptions.Unknown('timeout'),
         ],
     )
     @mock.patch.object(quantum, 'QuantumEngineServiceAsyncClient', autospec=True)
@@ -403,7 +404,6 @@ async def test():
             google_exceptions.TooManyRequests('too many requests'),
             google_exceptions.Unauthenticated('unauthenticated'),
             google_exceptions.Unauthorized('unauthorized'),
-            google_exceptions.Unknown('unknown'),
         ],
     )
     @mock.patch.object(quantum, 'QuantumEngineServiceAsyncClient', autospec=True)