Unhandled Gitlab::Git::CommitErrors in OperationService RPC's

For example https://sentry.gitlap.com/gitlab/gitaly-production/issues/159883/

I think we should follow the example of UserFFBranch and translate CommitError into FailedPrecondition. On the client side we can then turn FailedPrecondition into CommitError again.

Starting with GitLab 10.7, FailedPrecondition errors will no longer go into Sentry. Besides that, I suspect that we are failing to pass these messages along to the client now.