fix(similarity): Return failure reason from multithread seer call (#77522)

Return failure reason from multithread seer call in the backfill
This allows the reason to be logged and the logic to skip projects with
gateway timeouts
[here](https://github.com/getsentry/sentry/blob/ff9a79b8945522173459675164ecbcd7e055405c/src/sentry/tasks/embeddings_grouping/backfill_seer_grouping_records_for_project.py#L248-L279)

Author: jangjodi

src/sentry/tasks/embeddings_grouping/utils.py

@@ -499,6 +499,7 @@ def process_chunk(chunk_data, chunk_stacktrace):
         for seer_response in seer_responses:
             if not seer_response["success"]:
                 aggregated_response["success"] = False
+                aggregated_response.update({"reason": seer_response["reason"]})
                 return aggregated_response
 
             aggregated_response["groups_with_neighbor"].update(