Added basic styling and messages to 404 and 500 pages

404 and 500 error pages now include the same layout as the rest of the application. When the app is running in PROD it will return a basic error message as configured in the messages file - in DEV mode the app will return the exception messages as normal.
Have thought about this some more.

These pages should just replicate [whatever the Digital Marketplace does for errors](https://www.digitalmarketplace.service.gov.uk/nothing).

To the user they are not two different websites, they're one website and the consistency of messaging should reflect this. Also, we then don't need to bother asking anyone for content on these pages.