fix(mkmr): properly handle rebasing on top of target_branch failing

This now throws a message along the lines of:

""" Rebasing the current branch on top of the remote branch failed! Please see the following git output for what exactly went wrong:

'Patch failed at 0001 community/libreoffice: test Resolve all conflicts manually, mark them as resolved with "git add/rm <conflicted_files>", then run "git rebase --continue". You can instead skip this commit: run "git rebase --skip". To abort and get back to the state before "git rebase", run "git rebase --abort".'

After sucessfully rebasing your branch please run mkmr again. """

fixes #22 (closed)