Move "Object not found" error logic to client

[seancolsen]

Current behavior

• ❌ Visiting a Database Page for a non-existent database renders a Django error page (Bad)

  http://localhost:8000/this_database_does_not_exist/

  

• ❌ Visiting a Schema Page for a non-existent schema renders a Django error page (Bad)

  http://localhost:8000/mathesar_tables/999

  

• ✅ Visiting a Table Page for a non-existent table renders a Svelte error page (Good)

  http://localhost:8000/mathesar_tables/1/999

  

Desired behavior

• All of these "object not found" types of errors should caught on the front end so that we can render a page with navigation.

[seancolsen]

This is still relevant.