Gracefully handle missing snac codes in local transactions

At present, when a user enters a postcode which for an authority without a snac code, they are shown a 404. This is very much an edge case, but there are some postcodes (eg. AL10 9AB) which experience this issue right now.

This changes this behaviour to show an error message with a link to the [find your local council](https://www.gov.uk/find-your-local-council) service.