Return Unavailable for PostUploadPack sidechannel errors
As Gitaly clients are configured to retry read-only requests upon
receiving Unavailable response, it is desirable to have them retry
the request if some sidechannel connection-related errors were
encountered.
This should help reduce the number of errors the clients run into when Gitaly is restarted.
Related to #5883 (closed)
Edited by Ahmad Sherif