Update & delete behavior not consistent with other adapters when record does not exist.

When a delete or update is executed against a non-existent record, the sails-mongo, sails-disk, and sails-memory adapters (and possibly others as well, though I haven't investigated any aside from the three I mentioned), no error is passed to the callback function.  sails-rest, however, passes an error back to the callback after receiving a 404 from the restful service.  This patch seeks to make the behavior of sails-rest consistent with the other adapters.