Instead of a simple check, looking for an associated field error at or
above the field, this method now more focused on finding the reason for
a failure when the field has no value.
This allows performing a more thorough search including cases when the
field error is at, above, or even below (e.g. non-null nested field
that bubbled up), in the end falling back on request errors (e.g.
failed response without any field errors).
Also, rename ResponseField to GraphQlResponseField and move to a
top-level class.
See gh-10
cdd8b67fed