Stop exceptions from breaking the watcher

[thewilkybarkid]

We're currently hitting https://github.com/pattern-lab/patternengine-php-twig/issues/34#issuecomment-383032335, where an invalid Twig template (possible while developing) breaks the watcher. Using --no-procs reveals the exception and the terminated process, otherwise it silently fails (the watch process continues, but nothing happens).

This catches any exception and outputs the message.