[BUGFIX beta] Provide helpful error even if modelClass isn't found.

Previously, you would receive an error if no model was found corresponding to
the route name that you are in. We were already providing a nice assertion
(which lets the user know that the model hook wasn't defined and can't be
inferred), but immediately after that we would throw an error because we were
still calling `find` on a non-existent model factory.

This simply short curcuits the `find` call if `modelClass` isn't found.

Resolves #4109.