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
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fd5ffa425dfbc66db82980e7eb17a28c11c7fd37

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