Make it so the parent resolver returns a failed field named `foo` as
`__error_foo` containing the error message. This field can then be
picked up by the child resolver in order to produce a local error.
Moreover, this fixes the test for writeonly tokens, which used to create
a table in a db, and then try querying from a separate db, which ended
up not testing what it was supposed to at all.
I also changed the way mutations are guarded (the end result is pretty
much the same)
- I realised by PK querying doesn't need arguments like filter, limit or
offset, so I removed the argument name collision logic I added in the
last commit
- I implemented the actual logic for querying by PK. The code is a lot
shorter due to the new introspection system
- I moved the normal queries to the new introspection system for
consistency.
- I moved the logic of "coerce columns without a datatype" away from the
resolver and into the "sql -> gql" converter as to be able to reuse the
default introspection resolver which simply looks up fields into the
parent resolver's result.
- I added some comments documenting the Introspection.Resolver stuff and
it's behaviour
