Fix issue #66

[rlue]

Decided not to rest on my laurels and just went ahead and fixed it.

WARNING: I'm new to wemux and haven't tested all the edge cases. All I did was go through and replace all non-zero (and non-1) return statements with exit, and redirect any preceding echos to STDERR. There were a couple return 1 statements that are only function returns and shouldn't exit out of the program (as far as I can tell).

Please give this a very critical eye before merging. Thanks for an awesome utility.