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
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master

Details
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https://git.postgresql.org/pg/commitdiff/a69e041d0c91759fb60ab52e7e21e3ec3752c69b

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