Return error HTML page if visiting non-existent route

In the build there's a simple page called `50x.html` that's intended for use as an error page to return to the user if they visit a non-existent route. We currently don't make use of this in the nginx config: https://github.com/DiamondLightSource/TomoHub/blob/8697a2af2fe374fa29c2ee1b28180be680aeeba4/frontend/nginx.conf#L1-L23

so at the moment, nothing is returned to the user's web browser (the page is blank). Returning this error page would be good to do.

Inspect frontend/nginx.conf at the linked lines and locate the built 50x.html page. Start by checking how non-existent routes are handled, then verify in a browser that such a route returns the error page instead of a blank response.

Written by the indexing model from the issue text.