Do a better job of checking the supplied year is valid

[Moohan]

This should give a good error.

slfhelper::read_slf_episode("2022", "recid")

This code should be doing this but doesn't for some reason: https://github.com/Public-Health-Scotland/slfhelper/blob/production/R/format_year.R.

Ideally, we would have a super helpful error message along the lines of: error in read_slf_ep() the year “2022” isn’t a valid year, did you mean ‘2021’ or ‘2122’?”.