AsyncResult.legacyPropagateError

If res is a #GSimpleAsyncResult, this is equivalent to [gio.simple_async_result.SimpleAsyncResult.propagateError]. Otherwise it returns false.

This can be used for legacy error handling in async *_finish() wrapper functions that traditionally handled #GSimpleAsyncResult 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.