[webui] if no user or no password are given, give an error

src/webui/app/controllers/user_controller.rb

@@ -50,7 +50,8 @@ def do_login
       flash[:success] = "You are logged in now"
       redirect_to params[:return_to_path] and return
     end
-    redirect_back_or_to :action => 'login'
+    flash.now[:error] = "Authentication failed"
+    redirect_to :action => 'login'
   end
 
   def save