legacy_propagate_error

If this 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.

Parameters:

this	a [iface@Gio.AsyncResult]

Returns:

`TRUE` if throws is has been filled in with an error from this, `FALSE` if not.