continue watching after errors (fixes #145)

Creates a new watchify instance for each attempt (this is the method used by the watchify command line tool -- https://github.com/substack/watchify/blob/master/bin/cmd.js)

I attempted to leave existing code in place as much as possible, but it's getting a little convoluted and could probably bear being refactored at some point.

Apologies for any stupidities; I'm new to node/grunt/browserify