Improve error messages from

    the following documented members are not in the declaration: a
    the following documented members are not in the declaration: aa, bb

to the more concise

    documented member 'a' does not exist
    documented members 'aa', 'bb' do not exist

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-19-armbru@redhat.com>