[PATCH] Improve REPACK (CONCURRENTLY) error messages for unsupported configurations

Hi,

While exploring the new REPACK (CONCURRENTLY) feature, I noticed
a few user-facing error paths that could be made more accurate.
Patch series of three:

0001 -- When wal_level < replica, REPACK (CONCURRENTLY) currently
surfaces generic "replication slots ... wal_level" error
from CheckSlotRequirements(), with a CONTEXT line referring
to an internal worker. Add an upfront check that reports a
REPACK-specific error.

0002 -- check_concurrent_repack_requirements() reports the same
generic "no identity index" error for several distinct
cases, two of which are misleading: REPLICA IDENTITY FULL
(which is set, but the hint says there is no identity), and
a deferrable PK as the only identity (skipped per commit
832e220d99a, but the hint suggests adding an index that
already exists). Distinguish these cases.

0003 -- Four ereport(ERROR) calls in the REPACK CONCURRENTLY code
path lack errcode() and default to ERRCODE_INTERNAL_ERROR.
Add appropriate errcodes; in particular, the
apply_concurrent_update/delete failures map cleanly to
ERRCODE_T_R_SERIALIZATION_FAILURE.

All three are error-path only; the success path is unchanged. Each
patch is independently committable. Detailed rationale is in the
individual commit messages.

Tested with `make check` (245/245 pass) and `make isolation/check`
(128/128 pass) on top of master; 0002 also updates
src/test/regress/expected/cluster.out to match the new deferrable-PK
message.

Thanks,
Baji Shaik