fix throw exception condition when setting relationship active

In CRM_Contact_BAO_Relationship::setIsActive() we make a relationship API call and then condition whether to throw an exception error based on the error_message returned. But the error message has changed as a result of this commit, so it's no longer conditioning properly: https://github.com/civicrm/civicrm-core/commit/7ecf62753884d9c9ae982f458023ea76d4d408e7