better presentation for register new account

- error message when no user, pass or server given
- server is now automatically updated when jid change in connection category

sat.tac

@@ -384,8 +384,15 @@ class SAT(service.Service):
 
     def registerNewAccountCB(self, id, data):
         user = jid.parse(self.memory.getParamA("JabberID", "Connection"))[0]
+        password = self.memory.getParamA("Password", "Connection")
         server = self.memory.getParamA("Server", "Connection")
 
+        if not user or not password or not server:
+            info ('No user or server given')
+            #TODO: a proper error message must be sent to frontend
+            self.actionResult(id, "ERROR", {'message':"No user, password or server given, can't register new account."})
+            return
+
         confirm_id = sat_next_id()
         self.__private_data[confirm_id]=id