Implement a 404 error when a page doesn't exist

**Is your feature request related to a problem? Please describe.**
Currently there is no 404 error page so the user may not realise they have visited an invalid url, the page loads as if nothing is wrong.

**Describe the solution you'd like**
Implement a 404 error when a page doesn't exist

**Describe alternatives you've considered**
N/A

**Additional context**
N/A