Catch CommitErrors while rebasing

Zeger-Jan van de Wegrequested to merge
zj-catch-commit-error-user-rebase into master

In Sentry we got reports of Gitlab::Git::CommitError occuring while rebasing. This is nothing Gitaly can do anything about, so we catch this as FailedPrecondition, the gRPC status code.

On the client side this error should be rescued from and turned into an Gitlab::Git::CommitError again.

Closes #1137 (closed)

Internal links; not exhaustive list of sentry errors:

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