Handle invalid city names more appropriately

[rstrange-scottlogic]

Currently if you put in a city name that doesn't exist you are sent to a page with no data on it.

Ideally we should be doing some simple validation in our API and returning 404 if the city is invalid. The UI would then show a basic error page.

Acceptance Criteria:

• If you visit the city page for a city that does not exist you are shown an error page.
• The API returns 404.