pgsql: Update query cancel message: errmsg("canceling query due to

Log Message:
-----------
Update query cancel message:

errmsg("canceling query due to user request or statement timeout")));

Modified Files:
--------------
pgsql/src/backend/tcop:
postgres.c (r1.458 -> r1.459)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.458&r2=1.459)