Fix REPACK decoding worker not cleaned up on FATAL exit

Fix REPACK decoding worker not cleaned up on FATAL exit

When the launching backend of REPACK (CONCURRENTLY) is terminated via
pg_terminate_backend(), ProcDiePending causes ereport(FATAL) which
bypasses PG_FINALLY blocks.  As a result, stop_repack_decoding_worker()
is never called, leaving the decoding worker running indefinitely and
holding its temporary replication slot.

Fix by using PG_ENSURE_ERROR_CLEANUP, which handles both ERROR and
FATAL exits.

Author: Baji Shaik <baji.pgdev@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/CA+fm-RNoPxL2N7db_A0anMXV_aDu6jWj4PNOPtMtBUAPDPvSXQ@mail.gmail.com