Respond with 404 for plugins that return empty data

[codemonkey800]

There's an issue on the frontend and backend right now where calling the API with a plugin is not indexed will return empty data. We should update both the frontend to display a 404 page, and the backend to respond with a 404 status code if the plugin object is empty

Backend

Frontend

[codemonkey800]

This is how our current 404 pages look like, they're very basic 😆

[richaagarwal]

I've taken care of the backend portion in #1226

@codemonkey800 Updating the current 404 page seems less critical, it's basic but I think it does the job? Let me know if you still think frontend work is needed for this.

[richaagarwal]

This is working on staging as expected (e.g. https://staging.napari-hub.org/plugins/foobar) and pending a production release.