Don't trigger a sentry error for Gitaly exceptions

**Problem**

We add a Sentry error for cases when Gitaly is not available. We
already handle this exception, so Sentry tracking is not necessary.

We return a 200 response when the Gitaly is not available. It is
confusing and makes it difficult to track unsuccessful requests.

**Solution**

* Respond with 503 error
* Don't log exceptions in Sentry

Changelog: changed