It seems you are trying to generate Java classes using a WSDL from an HTTPS URL. It fails when it is trying to validate the certificate.
Try to access the WSDL from your browser and it should give you a warning.
You can save the WSDL to your local directory and try to run WSDL2Java for that file.
However you might get similar errors when you access the web service via stub, if the endpoint is also an HTTPS URL.