Better handle and test errors returned from the 3rd party service

We should better assert errors returned by 3rd party services like GCS (google cloud storage).
Currently we'll just get a 500 status and the returned error in the logs, but maybe we should return a better message to the caller.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-995)

No file, test, or entry point is named; start by tracing how the server handles errors from GCS and how those errors become HTTP 500 responses. Review existing error-handling tests, then define the caller-facing error behavior and add coverage showing that third-party failures return the intended message rather than only appearing in logs.

Written by the indexing model from the issue text.