Report the set of missing or unknown keys. And give a hint about the
accepted keys.

The error message for multiple meta type members (visible in
tests/qapi-schema/double-type.err) is not improved.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20181213123724.4866-6-marcandre.lureau@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>