[packager] Improve error message when branch could not be created

[simoncozens]

If the git branch command failed for any reason, you would get the error 'NoneType' object has no attribute 'name' when the code tried to use the (None) branch variable. This gives you the output from git.

[m4rc1e]

Looks good. However, I'm interested in understanding why it wasn't able to create the branch?

[simoncozens]

I've worked out why this was failing for me: I have git configured to pull with rebase instead of merge, to avoid needless merges. But that only works if the working tree is clean. _create_git_branch does a git pull --force but we are guaranteed to have a dirty working tree as we've just been editing a METADATA.pb file in there. So the git pull fails.