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_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/cdbb3921308ec71e7d090b734a1c8c0bf3269088

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