Improve REPACK (CONCURRENTLY) error messages some more

Improve REPACK (CONCURRENTLY) error messages some more

We had discussed changing the wording of messages from "cannot repack
table X" to "cannot execute REPACK on table X", so that translators
don't have to figure out how to translate REPACK as a verb in their
language.  We already do that for VACUUM and others and it's not very
nice.  Also remove extra double-quotes in a message of that form which I
mistakenly added in commit 43649b6a53e9.

While at it, add specific error messages for the cases of a table with a
deferrable primary key, and of REPLICA IDENTITY FULL; otherwise the user
gets a message that the table doesn't have an identity index and it's
not clear why that is.

Author: Baji Shaik <baji.pgdev@gmail.com>
Discussion: https://postgr.es/m/CA+fm-ROdgh0rEVuXoViBk4TVgjodrN=MTR_RYuOuKLZ9voX4YA@mail.gmail.com
Discussion: https://postgr.es/m/CABV9wwOo=wvq1hwTRK6HgBWUB=ekzsEebY30EWoc1V9UJQrrrw@mail.gmail.com