Sigtest should print more understandable error messages

For example the tool prints (format plain) :

Definition required but not found in javax.xml.ws.Service$Mode
    CLASS public static !enum javax.xml.ws.Service$Mode
Definition found but not permitted in javax.xml.ws.Service$Mode
    CLASS public final static !enum javax.xml.ws.Service$Mode

But actually it means only one error - "final" modifier found in tested API but not found in signature file.