res is a
GSimpleAsyncResult, this is equivalent to
g_simple_async_result_propagate_error(). Otherwise it returns
This can be used for legacy error handling in async *_finish()
wrapper functions that traditionally handled
error returns themselves rather than calling into the virtual method.
This should not be used in new code;
GAsyncResult errors that are
set by virtual methods should also be extracted by virtual methods,
to enable subclasses to chain up correctly.