Adjust error message

We usually don't use "namespace" in user-facing error messages.  Also,
in master this was replaced by another error message referring to
"temporary objects", so we might as well use that here to avoid
introducing too many variants.

Discussion: https://www.postgresql.org/message-id/bbd3f8d9-e3d5-e5aa-4305-7f0121c3fa94@2ndquadrant.com