... gimp_procedure_new_return_values().
When we see how this has been used in help or goat-exercise plug-ins so
far, we clearly see that we expect this function to be used as last call
in a run callback. Well we could save the result, free the error, then
return the result, but it's cumbersome.
So instead let's officially expect gimp_procedure_new_return_values() to
take ownership of the GError (i.e. it will free it in the end).
For this reason, remove the `const`.