Fixed andThenM to stop at first ExitFailure. Added test case to demonstrate it.

Hi Mark,

I think you might have missed the last comment I made to the last pull request. I found a bug in my implementation of andThenM. I had assumed that because andThen stopped at the first failure, that liftM2 andThen would too. Anyway, here's the fix along with a test case. This also reveals an inconsistency (bug?) in bash.

Warren