pgsql: Be more specific about OOM in XLogReaderAllocate

Be more specific about OOM in XLogReaderAllocate

A couple of spots can benefit from an added errdetail(), which matches
what we were already doing in other places; and those that cannot
withstand errdetail() can get a more descriptive primary message.

Author: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Julien Rouhaud <rjuju123(at)gmail(dot)com>
Discussion: https://postgr.es/m/CALj2ACV+cX1eM03GfcA=ZMLXh5fSn1X1auJLz3yuS1duPSb9QA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2fed48f48f7f2f7a6d6f6d020f046efe3c249828

Modified Files
--------------
src/backend/access/transam/xlog.c | 2 +-
src/backend/replication/logical/logical.c | 3 ++-
src/backend/replication/walsender.c | 3 ++-
src/bin/pg_rewind/parsexlog.c | 6 +++---
src/bin/pg_waldump/pg_waldump.c | 2 +-
5 files changed, 9 insertions(+), 7 deletions(-)