file watching: replace transform-based dependency tracking with proxied bundle.deps

Reasoning: browserify.transform does not trigger for package/node_modules-based requires (e.g. require('engine.io-client')), so those files do not get watched. By replacing browserify.deps with a proxied version before calling browserify.bundle, we can get complete list of dependencies (trade-off: if the browserify api changes, this might break).
[API change!] pass bundling errors to watchCallback