Make Ember.Error available in ember-metal.

This allows us to use `Ember.Error` from within the `ember-metal` package.  

The only instances of `throw new Error` left after this are in the following vendored micro libraries:

* `packages/container/lib/main.js`
* `packages/ember-metal/lib/vendor/backburner.js`
* `packages/ember-routing/lib/vendor/route-recognizer.js`
* `packages/ember-routing/lib/vendor/router.js`
* `packages/loader/lib/main.js`
* `packages/metamorph/lib/main.js`