Improve disambiguation error message

[steve-chavez]

As noted in #1358 (comment) we had the relationships backward(getting the cardinality from parent to child) on the request tree. This made things really confusing, not only for development/debugging but also for error reporting. With this change, we now get a better error message for disambiguation(added in #1401).

GET /message?select=*,sender(*)

{
  "details": [
    {
        "cardinality": "many-to-one",
        "source": "test.message[sender]",
        "target": "test.person[id]"
    },
    {
        "cardinality": "many-to-one",
        "source": "test.message[sender]",
        "target": "test.person_detail[id]"
    }
  ],
  "hint": "Disambiguate by choosing a relationship from the `details` key",
  "message": "More than one relationship was found for message and sender"
}

Also renamed the cardinality types from Parent/Child to the more proper O2M/M2O. Parent and Child terms are usually meant for referring to the ends of a relationship and not the cardinality.