Improve Git HTTP err message given when Gitaly unavailable

What does this MR do and why?

Currently, we receive either:

Cloning into 'blank'...
remote: 14:failed to connect to all addresses. debug_error_string:{"created":"@1632427661.133230000","description":"Failed to pick subchannel","file":"src/core/ext/filters/client_channel/client_channel.cc","file_line":3952,"referenced_errors":[{"created":"@1632427661.133227000","description":"failed to connect to all addresses","file":"src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc","file_line":394,"grpc_status":14}]}
fatal: unable to access 'http://127.0.0.1:3000/root/blank.git/': The requested URL returned error: 503

Let's change it to:

Cloning into 'blank'...
remote: Git service is temporarily unavailable
fatal: unable to access 'http://127.0.0.1:3000/root/blank.git/': The requested URL returned error: 503

Related issue: #340960 (closed)