Send BadRequest for unsupported qualifiers

[Qinusty]

Relevant part of the spec.

  // In the case of unsupported qualifiers, the server *SHOULD* additionally
  // send a [BadRequest][google.rpc.BadRequest] error detail where, for each
  // unsupported qualifier, there is a `FieldViolation` with a `field` of
  // `qualifiers.name` and a `description` of `"{qualifier}" not supported`
  // indicating the name of the unsupported qualifier.