Exit with an error message if git checkout $DEV_BRANCH fails.

This can happen for instance in stacked backports, if the user
didn't answer 'Y' when prompted to create a backport branch
during the backport of the previous patch.

Change-Id: I5ba0929c15727a904fdaae14880eada49600428f
1 file changed
tree: fc5c64cbff65b16cbbe62ae69110798fecb93cbe
  1. backflip