pgsql: When restoring GUCs in parallel workers, show an error context.

When restoring GUCs in parallel workers, show an error context.

Otherwise it can be hard to see where an error is coming from, when
the parallel worker sets all the GUCs that it received from the
leader.  Bug #15726.  Back-patch to 9.5, where RestoreGUCState()
appeared.

Reported-by: Tiago Anastacio
Reviewed-by: Daniel Gustafsson, Tom Lane
Discussion: https://postgr.es/m/15726-6d67e4fa14f027b3%40postgresql.org

Branch
------
REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/89a3cdb32a9e1a6545abd43ef385e6bb91d66cb9

Modified Files
--------------
src/backend/utils/misc/guc.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)