It is important that if a program recieved ^C from the user, and the
program terminates as a result, that it exit with SIGINT so that the
parent process knows that was the case. For example cabal calling ghc
will then be able to tell the difference between it failing and the user
interrupting with ^C and report (or not) appropriately.

So we should just let the UserInterrupt exception propagate to the top
level error handler which will do the right thing.

See http://www.cons.org/cracauer/sigint.html