This trivial patches fixes the error message returned by CREATE INDEX

This trivial patches fixes the error message returned by CREATE INDEX
when it finds an existing relation with the same name as the
to-be-created index.

Old error message:

nconway=# create table foo (a int);
CREATE TABLE
nconway=# create index foo on foo (a);
ERROR:  index named "foo" already exists

I replaced 'index' with 'relation' in the error message.

Neil Conway