Use sh -e to make sure we abort if something goes wrong.

Summary:
I was using 'git rb' today and the first 'git co' failed, but the
script continued, and as a result it hung the review-branch off the
wrong deploy-branch.  Confusion everywhere!  Now we'll give an error.

Test Plan: Fingers crossed, mostly.

Reviewers: desmond, alpert, benkomalo

Reviewed By: alpert, benkomalo

Differential Revision: http://phabricator.khanacademy.org/D13793