Added proxy identifier values to the EntityNotFoundException

It would be very nice if the identifier values are included in the EntityNotFoundException. That information saves a whole lot of time finding the missing association in the database. 
@pietervogelaar please look at https://github.com/doctrine/doctrine2/blob/f5ecabbc2198973f5e54cb76e99709c59fe62ffa/lib/Doctrine/ORM/EntityNotFoundException.php#L35: I'd rather add a `public static function fromInvalidProxy(ClassMetadata $classMetadata, $entity)` or such