Implement proper 404 on entities

## Describe the bug
In case there is a 404 and entity was not found, implement a proper 404 instead of telling the user to try again in a few minutes. What is more, the URL reverts to /dataset (or schema, or mapping table) and view to the first element of that entity type.

And focus on an item in the navigation menu lingers as per #650

## To Reproduce
Steps to reproduce the behavior OR commands run:
1. Go to any dataset
2. Append any char [a-zA-Z0-9] to the URL
3. Press enter

## Expected behaviour
I expect a message that tells me it was not found in the database and the URL to revert back to the previous one.

## Screenshots