evdev: warn about missing input devices

evdev: warn about missing input devices

Usually there should be at least one input device, when Weston starts
up, or is reactivated by a VT switch. Add a nice warning, in case there
are no input devices.

This is to give a clue to users who happen to try Weston on DRM, and
do not get any response.

Add also a message to another failure case, that may lead to missing
input devices.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>