app: gimp_procedure_execute(): don't use a NULL plug-ins error message

return_vals[1] being present and being a string doesn't neccessarily
mean it comes from the plug-in, it might just as well be its first
return value, which gets generated and NULL-initialized by the plug-in
execution code even if the plug-in didn't actually return anything.

(cherry picked from commit 60014630fa732365e03683bb31ac6a2b2511fe79)