To improve the code, move the error check in logical_read_xlog_page().

To improve the code, move the error check in logical_read_xlog_page().

Commit 0fdab27ad6 changed the code to wait for WAL to be available before
determining the timeline but forgot to move the failure check.

This change is to make the related code easier to understand and enhance
otherwise there is no bug in the current code.

In the passing, improve the nearby comments to explain why we determine
am_cascading_walsender after waiting for the required WAL.

Author: Peter Smith
Reviewed-by: Bertrand Drouvot, Amit Kapila
Discussion: https://postgr.es/m/CAHut+PvqX49fusLyXspV1Mmd_EekPtXG0oT146vZjcb9XDvNgw@mail.gmail.com