function onBundle( error, bundle ) {
		if ( error ) {
			debug( 'Encountered an error when generating a browserify bundle: %s', error.message );
			return done( error );
		}
		debug( 'Successfully generated a browserify bundle.' );
		done( null, bundle );
	}
Callback invoked upon generating a browserify bundle.

@private
@param {(Error|null)} error - error object
@param {(string|Buffer)} bundle - bundle
@returns {void}