Fix bogus coding in ExecAppendAsyncEventWait().

Fix bogus coding in ExecAppendAsyncEventWait().

No configured-by-FDW events would result in "return" directly out of a
PG_TRY block, making the exception stack dangling.  Repair.

Oversight in commit 501cfd07d; back-patch to v14, like that commit, but
as we do not have this issue in HEAD (cf. commit 50c67c201), no need to
apply this patch to it.

In passing, improve a comment about the handling of in-process requests
in a postgres_fdw.c function called from this function.

Alexander Pyhalov, with comment adjustment/improvement by me.

Discussion: https://postgr.es/m/425fa29a429b21b0332737c42a4fdc70%40postgrespro.ru