Don't create a branch if there are errors executing the release spec

[mcmire]

If you run the tool and it executes the release spec, but there are errors — for instance, perhaps there are packages you ought to include that you haven't yet — the tool will abort, but it will still create the release branch and place you on it. If you decide to rename this branch or change the branch somehow, then you update the release spec and re-run the tool so that it re-reads the release spec, the tool won't know that changed the branch and will switch you back to the original branch (or recreate it if necessary).

To avoid that from happening, instead of creating the release branch in the first place, the tool should delete it if it detects an error.