Exit process if error passed to broadcast

Fixes #519 
What exactly would we be testing for? Do we just test that certain error(s) cause the program to gracefully exit? 

We originally got this trying to use XBee. We'd get a ready event even though the XBees weren't actually talking. But my code assumed all was fine because it let me proceed.

Some questions to think about:
1) How do we reproduce such a scenario in the tests?
2) What other potential errors are broadcast that we'd want to test they also gracefully exit?
3) Are there types of errors passed to broadcast() that we DON'T want to fully exit the process?