Add more helpful error for non-response in Action

Part of #460

There is still a bug that makes it so we can't show the type, but I
think we should show this error without the type. It is still better
than what is there.

src/lucky/renderable.cr

@@ -103,6 +103,22 @@ module Lucky::Renderable
     response.print
   end
 
+  private def handle_response(_response : T) forall T
+    {%
+      raise <<-ERROR
+
+      #{@type} returned #{T}
+
+      But it should return a Lucky::Response.
+
+      Try this...
+
+        ▸ Use a method like render/redirect/json at the end of your action.
+        ▸ Ensure conditionals (if/else/case) at the end of your action return a response (render/redirect/json/etc.)
+      ERROR
+    %}
+  end
+
   private def log_response(response : Lucky::Response) : Nil
     response.debug_message.try do |message|
       Lucky.logger.debug(message)