Better user feedback upon refusal of push because of protected branch

Protection of branches is enforced by the internal pre-receive Git hook. It has a user feedback, which is intended to be displayed in the Git client (would normally be the end user), but that gets swallowed in Heptapod setup.

Find a way to log it and if possible display within the Hg client.

For instance, protected branches can't be force-pushed and that can be a problem in cases of multiple heads conditions, or if pruning once all heads are bookmarked (happened to me on this very repository for branch/default once we moved to 0.7.dev1).

I suspect that would made bugs like #109 (closed) easy to work around or at least to understand.

Side notes:

• there is a level of automatic protection for the default branch in the global admin settings, by default it is protected
• GitLab 10.3 creates a new level of protection in which only merges from GitLab are accepted (stumbled on that while running the functional tests against the gitlab-10.3 topic)