[API] Catch rate limit errors on repo auth

[trent-codecov]

Catch shared.torngit.exceptions.TorngitRateLimitError during repo auth

1. We want to raise an exception and return a 429 status code.
2. Emit a Sentry metric for this error condition (Example)

The current behavior is an uncaught exception. As this is a known error condition, we should treat it as such.

Add an error message to allow us to differentiate a GitHub rate limit from a Codecov rate limit.