[BUGFIX beta] Provide helpful error even if Model 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 have 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.
Merge pull request #4208 from rjackson/allow-missing-store-in-model-hook

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