Add more helpful error for non-response in Action

[paulcsmith]

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.

Currently:

in src/lucky/renderable.cr:98: no overload matches 'Rendering::Index#handle_response' with type (Lucky::Response | Nil)
Overloads are:
 - Lucky::Renderable#handle_response(response : Lucky::Response)

    handle_response(response)
    ^~~~~~~~~~~~~~~

Rerun with --error-trace to show a complete error trace.

With this change:

in src/lucky/renderable.cr:98:
Your action returned (Lucky::Response | Nil)

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.)

    handle_response(response)

Still not a helpful trace as the file is not the user's file, but at least it explains what to do a bit better