Consolidate internationalized server-side error message text

[toolness]

In #1895 we made some of our rare edge-case error messages more human-friendly and internationalized them, but only for EFNY. Identical error messages exist in other parts of our code though, e.g. norent. We should find those error messages and make sure they use the same text as EFNY--perhaps we should even consolidate some of the validation logic itself to make things more DRY.