Praefect should not emit Gitaly errors to Sentry

When Praefect proxies request to Gitaly and receives error as a result Sentry interceptor catches it and create a new event, so it is a duplicate of the error that happened in Gitaly. To omit this we explicitly mark such errors using context and gRPC tags. However it is not always possible to distinct between Gitaly errors and system errors, that is why we rely on the assumption about presents of the trailers in the response.

Closes: #2434 (closed)