Don't trigger a sentry error for Gitaly exceptions

What does this MR do and why?

Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3198686

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 (similar to TagsController)
• Don't log exceptions in Sentry

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.