legacyPropagateError

If @res is a class@Gio.SimpleAsyncResult, this is equivalent to method@Gio.SimpleAsyncResult.propagate_error. Otherwise it returns FALSE.

This can be used for legacy error handling in async *_finish() wrapper functions that traditionally handled class@Gio.SimpleAsyncResult error returns themselves rather than calling into the virtual method. This should not be used in new code; iface@Gio.AsyncResult errors that are set by virtual methods should also be extracted by virtual methods, to enable subclasses to chain up correctly.