Fixed error handling

Handling only error code 127 and ignoring anything else seems a bit careless to me, and a bit of wasted effort - what does Grunts 'done' callback take an error argument for anyway? What if composer cowardly refuses to install my dependencies? It's silently failing and I get unrelated errors much later in the build...

Fixed that - just pass on the error, fail the build and let the user sort out whatever Composer is complaining about.