Emit namespace in the post-copy errmsg

Emit namespace in the post-copy errmsg

During a VACUUM or CLUSTER command, the initial output emits a
fully qualified relation path with namespace.  The post-action
errmsg only emitted the relation name however, which may lead
to hard to parse output when using multiple jobs with vacuumdb
as the output from different jobs may be interleaved.  Include
the full path in the post-action errmsg to be consistent with
the initial errmsg.

Author: Mike Fiedler <miketheman@gmail.com>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://postgr.es/m/CAMerE0oz+8G-aORZL_BJcPxnBqewZAvND4bSUysjz+r-oT1BxQ@mail.gmail.com