Handle non-existent and protected routes

Currently accessing any route which doesn't exist opens up a blank view with the navbar.

- [ ] Implement a 404 page to catch all non-existent routes.
- [ ] Implement a 403 forbidden page to catch protected routes which are unlocked only when certain conditions are fulfilled (for example transaction confirmation requires like of validators, budget + authorization from PolkadotJS wallet and Suggested validators needs a valid budget input, et al.)