Don't return success if Frege compilation fails

[earldouglas]

e.g.

1. Waiting for source changes... (press enter to interrupt)
E /home/james/simple/src/main/frege/com/earldouglas/helloworld/HelloWorld.fr:6: unexpected '}' while trying to parse a variable name
E /home/james/simple/src/main/frege/com/earldouglas/helloworld/HelloWorld.fr:6: syntax error on end of file
runtime 0.969 wallclock seconds.
[success] Total time: 1 s, completed Jul 30, 2015 3:54:06 PM