pgsql: Improve logical replication connection-failure messages.

Improve logical replication connection-failure messages.

These messages mostly said "could not connect to the publisher: %s"
which is lacking context. Add some verbiage to indicate which
subscription or worker process is failing.

Nisha Moond

Discussion: https://postgr.es/m/CABdArM7q1=zqL++cYd0hVMg3u_tc0S=0Of=Um-KvDhLony0cSg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d8bd0a72ea284ffb1d1154efbe799241cc5edc6

Modified Files
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src/backend/commands/subscriptioncmds.c | 9 ++++++---
src/backend/replication/logical/slotsync.c | 3 ++-
src/backend/replication/logical/tablesync.c | 3 ++-
src/backend/replication/logical/worker.c | 3 ++-
src/backend/replication/slotfuncs.c | 3 ++-
src/backend/replication/walreceiver.c | 9 +++++----
src/test/regress/expected/subscription.out | 2 +-
7 files changed, 20 insertions(+), 12 deletions(-)