Warn and bail out when create or makeCurrent fails during init

Return null from getContext when this happens. These two places are
candidates for failing. Also warn so we have a chance to see where
we fail.

Change-Id: I6a5c8c1d229b16d8c0d9705d4595b40022540aaf
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>