Implement logic to correctly handle 404 and 500 error

[MarcusNoble]

Currently Django seems to be throwing any problem as a 400 bad request. Missing pages and server exceptions need to be handled separately.

404.html and 500.html also seems to show the login link instead of the menu options. Need some way of passing in the authenticated user.

This may be to do with ALLOWED_HOSTS in settings.py

Also need to handle additional status codes (such as 403)

For Tester:

Before testing this ticket, please check with me that the server isn't in Debug mode (as this bypasses all custom error pages).

1. Navigate to ​http://datareducedev.isis.cclrc.ac.uk/autoreduce_webapp/notaurl
2. Verify a custom 404 page is display
3. If you aren't down as an instrument scientist/admin navigate to: ​http://datareducedev.isis.cclrc.ac.uk/autoreduce_webapp/instrument/GEM/
4. Verify a custom forbidden page is shown, similar to the 404 page.

[MarcusNoble]

(In #10286) Note: The logic for handling 404 and 500 errors is not currently working. See #10318

[Lottie Greenwood]

Both 404 and 403 page work as expected.