Draft: status: Return ResourceExhausted code if there are no more file descriptors

When gitaly has troubles with acquiring more file descriptors from the system it returns Internal or Unknown status codes back to the caller. That doesn't tell the caller anything meaningful, so the caller re-attempts the operation. Re-attempts create additional load on the gitaly. What we want instead is a backoff that will allow gitaly to handle in process requests and lower file descriptors usage. That is why now gitaly returns ResourceExhausted status code in case it encounters a problem with file descriptors.

Closes: #4688 (closed)