Added missing onPackComplete Signal

The asset pack loader references a missing "onPackComplete" Signal.  The loader fails when it attempts to dispatch the event.  This bug is currently reproduced in the [Asset Pack Example](http://examples.phaser.io/_site/view_full.html?d=loader&f=asset+pack.js&t=asset%20pack).

I've also updated "packError" to correctly send ``false`` to "nextPack" when a pack has failed to load.