Backpatch b61d161c14

I propose to backpatch b61d161c14 [1] (Introduce vacuum errcontext to
display additional information.). In the recent past, we have seen an
error report similar to "ERROR: found xmin 2157740646 from before
relfrozenxid 1197" from multiple EDB customers. A similar report is
seen on pgsql-bugs as well [2] which I think has triggered the
implementation of this feature for v13. Such reports mostly indicate
database corruption rather than any postgres bug which is also
indicated by the error-code (from before relfrozenxid) for this
message. I think there is a good reason to back-patch this as multiple
users are facing similar issues. This patch won't fix this issue but
it will help us in detecting the problematic part of the heap/index
and then if users wish they can delete the portion of data that
appeared to be corrupted and resume the operations on that relation.

I have tried to back-patch this for v12 and attached is the result.
The attached patch passes make check-world but I have yet to test it
manually and also prepare the patch for other branches once we agree
on this proposal.

Thoughts?

[1] -
commit b61d161c146328ae6ba9ed937862d66e5c8b035a
Author: Amit Kapila <akapila(at)postgresql(dot)org>
Date: Mon Mar 30 07:33:38 2020 +0530

Introduce vacuum errcontext to display additional information.

The additional information displayed will be block number for error
occurring while processing heap and index name for error occurring
while processing the index.

[2] - https://www.postgresql.org/message-id/20190807235154.erbmr4o4bo6vgnjv%40alap3.anarazel.de

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com