pgsql: Modify the parser's error reporting to include a specific hint

Log Message:
-----------
Modify the parser's error reporting to include a specific hint for the case
of referencing a WITH item that's not yet in scope according to the SQL
spec's semantics. This seems to be an easy error to make, and the bare
"relation doesn't exist" message doesn't lead one's mind in the correct
direction to fix it.

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.264 -> r1.265)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.264&r2=1.265)
pgsql/src/backend/parser:
parse_cte.c (r2.3 -> r2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_cte.c?r1=2.3&r2=2.4)
parse_relation.c (r1.138 -> r1.139)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_relation.c?r1=1.138&r2=1.139)
pgsql/src/include/access:
heapam.h (r1.138 -> r1.139)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/heapam.h?r1=1.138&r2=1.139)
pgsql/src/include/parser:
parse_node.h (r1.57 -> r1.58)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_node.h?r1=1.57&r2=1.58)