when compiler.js fails, it should also return an error code and give a u...

...seful error message (the printErr wasn't always doing that?)

For whatever reason, I was having a very hard time figuring out where a particular invocation of emcc was failing, and it turns out compiler.js was 1) not returning a nonzero exit code and 2) not printing a stack trace.

If I removed the catch clause, it did exactly what it should have, and made the problem easy to diagnose.