Better error correlation in sentry

Currently Sentry is using a stacktrace to find the "culprit" of Gitaly GRPC errors.

Unfortunately Go errors don't contain stack information, so sentry uses the reporting codes stack instead.

This means that gitlab.com/gitlab-org/gitaly/vendor/github.com/getsentry/raven-go in CaptureError is always set as the culprit.

The downstream knock-on effect is that our sentry errors are very badly correlated at present, so it's very difficult to figure out a fault from a sentry error, since multiple faults may correlate to the same sentry report.

This merge request fixes this problem by setting the GRPC service name and method name as the culprit.