[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.

Closes #5148.

1.6.1 will need to be released after this has been merged.
How does this behave on older browsers that don't have `Error#stack`?