new WikiI18nException, meant to carry key & args to construct i18n messages to show meaningful messages to end-user. Classes and/or JSPs on the web tier are responsible of constructing the appropiate message.

DuplicateUserException inherits from WikiI18nException.

git-svn-id: https://svn.apache.org/repos/asf/incubator/jspwiki/trunk@1439577 13f79535-47bb-0310-9956-ffa450edef68