Updated the error message for find to stay consistent across Rails

The ``RecordNotFound`` error message that is raised when using ``find`` on an association which has ``inverse_of`` defined does not match the error message that is thrown for non-inverse-of associations. 

I've changed the error message to stay consistent with the error thrown for ``find`` without an
inverse_of association.

I also removed some variables that were not needed in the test cases related to the ``RecordNotFound`` error messages.