error swapping formula branch

[munhitsu]

When swapping branch from master to say debug in master formula-requirements.txt for specific formula - salt-shaker can't cope with it. As new remote branch is not available locally.
rm -Rf vendors fixes the problem.

Would be usefull to either exend error message or check for branch on remote.

[sevenmachines]

Theres issues with the way we git checkout and update