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
------
master

Details
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https://git.postgresql.org/pg/commitdiff/3c8c55dd5445370c5cad3ae04de02caba7be7073

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