Prevent legacy pipeline from swallowing exceptions on checkout

Our legacy pipeline assumes that if any error occurs when checking out a branch and rebasing it on the base branch, that it must be because the branch does not exist. But this might not be the case. There might be some other error during checkout or even conflict when rebasing. This means that we can still bet a run flagged as successful in these cases. We should make sure that we handle the branch no found exception and re-throw any others.