pgsql: Improve vacuum error context handling.

Improve vacuum error context handling.

Use separate functions to save and restore error context information as
that made code easier to understand.  Also, make it clear that the index
information required for error context is sane.

Author: Andres Freund, Justin Pryzby, Amit Kapila
Backpatch-through: 13, where it was introduced
Discussion: https://postgr.es/m/CAA4eK1LWo+v1OWu=Sky27GTGSCuOmr7iaURNbc5xz6jO+SaPeA@mail.gmail.com

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/83fa48c8cd26c9a8171a85e786bb6ae1c5b04139

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c | 123 ++++++++++++++++++++---------------
src/tools/pgindent/typedefs.list     |   1 +
2 files changed, 73 insertions(+), 51 deletions(-)