worker to log more information for gitter internal server errors

There are a lot of 500 internal server errors observed from gitter in worker, probably from [affectedcommits.go](https://github.com/google/osv.dev/blob/master/go/internal/worker/affectedcommits.go#L94).

The current error message `gitter responded with 500 Internal Server Error` is not that helpful to investigate the issue, and we should consider log more information for this error.

Start in go/internal/worker/affectedcommits.go around line 94, where the worker reports Gitter's 500 Internal Server Error. Inspect the existing error-handling path and determine what response or request context is available for investigation. Done means the error log includes more useful information while still clearly identifying the Gitter failure.

Written by the indexing model from the issue text.