onFailed 
  fun onFailed(connectFlags: <Error class: unknown class> = ConnectFlags(0u), handler: (<Error class: unknown class>) -> Unit): <Error class: unknown class>(source)
This signal is emitted when an error occurs during the resource load operation.
Parameters
connectFlags 
a combination of ConnectFlags
handler
the Callback to connect. Params: error the #GError that was triggered