* build-aux/announce-gen (main): Erroneous usage would elicit a
misleading diagnostic. When --gnulib-version=V was specified:
- without --bootstrap-tools=..., or
- with --bootstrap-tools=..., but without "gnulib" in that list
we would give an inappropriate diagnostic. Now, each diagnostic
is tailored to the precise erroneous condition.
Reported by Bruno Haible in
https://lists.gnu.org/r/bug-gnulib/2022-01/msg00025.html