Pass errors in container.lookup to Ember.onerror.

This is meant as an initial proof-of-concept to enable `container.lookup` errors to be sent through the `Ember.onerror` pathway. It is likely that the final API/design should be different.

Adds `container.onerror` and defaults to `Ember.onerror`, also adds an `ember-testing` initializer to ensure that the container's `onerror` is set during testing.

Closes #3845.