Ensures, when relationships are specified, that the root is an object
and reach relationships contains a data member with a well-formed
resource linkage per:
* https://jsonapi.org/format/#crud-creating
* https://jsonapi.org/format/#crud-updating-resource-relationships
* https://jsonapi.org/format/#document-resource-object-relationships
* https://jsonapi.org/format/#document-resource-object-linkage

Uses a new `error_attrs` type to represent the raw output of
`build_error`. This is the output of some of the newer error functions.

Also includes an `error_attrs` suffix on those functions to highlight
that they return the raw `build_error` output structure instead of the
JSON:API-friendly error structure returned by most of the other
ErrorView functions.