Error code 10 is actually sent by your wifi module, that is why you only get this error when the module is hook on you pl2303 key. And it mean "Join fail" althouth the pl2303 does get hot when you hook the wifi module, it will never send any error message of any kind. It does not have what is required in it to send any error message.
Personnally, i don't use this kind of usb2serial any more cause it tend to become to hot, but it does the job still.