Better error printing from the rebar plugin

The problem is that cuttlefish assumes that lager is running for its logging.

That's always true, except when running the rebar plugin. Rebar doesn't have lager built in, and I'm not saying that it should. What this PR does is io:format errors IF errors are returned to the rebar_plugin. Any other path assumes they've been lager'd.

Also piggybacked in here a fix for vm_args that are being output as ~s even when they're integers. This is probably the little devil behind #48. In fact, I know it is because, spoiler alert, I've got a fix for that coming too.