getUser will throw UserNotFoundException instead of IllegalArgumentException for illegal usernames.

git-svn-id: http://svn.igniterealtime.org/svn/repos/messenger/trunk@812 b35dd754-fafc-0310-a699-88a17e54d16e

src/java/org/jivesoftware/messenger/user/UserManager.java

@@ -128,7 +128,7 @@ public class UserManager {
             username = Stringprep.nodeprep(username);
         }
         catch (StringprepException se) {
-            throw new IllegalArgumentException("Invalid username: " + username,  se);
+            throw new UserNotFoundException("Invalid username: " + username,  se);
         }
         User user = (User) userCache.get(username);
         if (user == null) {