Follow-up from "protocol: add wl_compositor.error.bad_parent"

The following discussion from !264 (merged) should be addressed:

• @pq started a discussion: (+1 comment)

  The first commit is good.

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  FWIW, Weston's test suite already checks that:

  • test_subsurface_paradox: making a surface into its own child produces BAD_SURFACE
  • test_subsurface_loop_paradox: creating a sub-surface loop with three surfaces produces BAD_SURFACE

The mentioned MR changes the required error code. Weston and its test suite needs to be updated.