This makes them compliant with the new version of RFC 401 (i.e.
RFC 1052).
Fixes#26391. I *hope* the tests I have are enough.
This is a [breaking-change]
I think I didn't run tests properly - my second call to
select_all_obligations_or_error has made 3 tests fail. However, this is
just an error message change - integer fallback never worked with casts.