Fix the display of UNKNOWN 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