pgsql: Fix the display of iso-8859-1 message type in apply worker.

Fix the display of UNKNOWN message type in apply worker.

We include the message type while displaying an error context in the
apply worker. Now, while retrieving the message type string if the
message type is unknown we throw an error that will hide the original
error. So, instead, we need to simply return the string indicating an
unknown message type.

Reported-by: Ashutosh Bapat
Author: Euler Taveira, Amit Kapila
Reviewed-by: Ashutosh Bapat
Backpatch-through: 15
Discussion: https://postgr.es/m/CAExHW5suAEDW-mBZt_qu4RVxWZ1vL54-L+ci2zreYWebpzxYsA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d38ad8e31dee1a69f4f6d4b6bb4e007e3751a93c

Modified Files
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src/backend/replication/logical/proto.c  | 13 ++++++++++---
src/backend/replication/logical/worker.c |  2 +-
src/include/replication/logicalproto.h   |  2 +-
3 files changed, 12 insertions(+), 5 deletions(-)