Replace ad hoc Ruby exceptions in proto with exception bridge
In #588 (closed) we are working on a uniform way to instantiate Ruby exceptions from Gitaly calls in gitlab-ce, and to "catch" them in gitaly-ruby.
If you look through GitalyClient in gitlab-ce you find a number of places where we instantiate custom exceptions based on special flags in the proto response messages etc. We should clean these up so that they all use the exception bridge.
There is no need to remove the ad hoc exception flags from the protocol, we can just stop sending the flag values.
We should take care to make this change in a backwards compatible way. We want that an 'old' client can talk to a 'new' server and vice versa.