Correct error message

The ValueError messages from _validate and _verify was not used.
While it's correct that AA12345678 is not a valid VAT the error message should be "AA is not a VIES member country."

The same happens if VIES is unavailable. The previous result from that was "XXXXXX is not a valid European VAT.". With this change the error is the more correct "VIES checkVat service unavailable.".