[BUGFIX release] Do not throw uncaught errors mid-transition.

The primary intent of this was to rethrow the error when it wasn't a
known (ignorable) type, and the default RSVP error handler would ensure
that it was printed to the console.

This worked wonderfully, except that it caused an error to occur (an
`Ember.assert` throws an error) during the rendering of the `error`
route (or template).

This changes from `throw`ing the error to just logging it, but the
userland result is the same: useful console error messages showing when
bad things happen in route hook promises.