pgsql: When an ERROR happens on a dblink remote connection, take pains

Log Message:
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When an ERROR happens on a dblink remote connection, take
pains to pass the ERROR message components locally, including
using the passed SQLSTATE. Also wrap the passed info in an
appropriate CONTEXT message. Addresses complaint by Henry
Combrinck. Joe Conway, with much good advice from Tom Lane.

Modified Files:
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pgsql/contrib/dblink:
dblink.c (r1.73 -> r1.74)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.73&r2=1.74)
pgsql/contrib/dblink/expected:
dblink.out (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out?r1=1.23&r2=1.24)