Convert
    visit_type_FOO(v, ..., &ptr, &err);
    ...
    if (err) {
        ...
    }
to
    visit_type_FOO(v, ..., &ptr, errp);
    ...
    if (!ptr) {
        ...
    }
for functions that set @ptr to non-null / null on success / error.
Eliminate error_propagate() that are now unnecessary.  Delete @err
that are now unused.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200707160613.848843-40-armbru@redhat.com>