stable/viewporter: add more error cases

Rather than silenty doing things, make them explicit and error if
anything is not quite right. Suggested by Daniel Stone.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
[Pekka: updated copyright years]
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>