pgsql: Improve error message for the case where a requested foreign key

Log Message:
-----------
Improve error message for the case where a requested foreign key constraint
does match some unique index on the referenced table, but that index is
only deferrably unique. We were doing this nicely for the
default-to-primary-key case, but were being lazy for the other case.

Dean Rasheed

Modified Files:
--------------
pgsql/src/backend/commands:
tablecmds.c (r1.296 -> r1.297)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.296&r2=1.297)