fix : with failOnError true, exit code is still 0 after error

Hi,

I'm using this plugin to run PHPUnit in a grunt task fired by a git pre-commit hook, and need the hook to fail if phpUnit fails. Currently even with failOnError set to true, the error code returned from grunt after a failed and aborted run is 0. This fix makes the task die spectacularly on fail and reports a non zero error code back to the shell.

Cheers,
Adam