client: read_events should return -1 after an error

When a thread is sleeping, waiting until another thread read
from the display, it always returns 0. Even when an error
occured. In documentation stands:

  "return 0 on success or -1 on error.  In case of error errno will
   be set accordingly"

So this is a fix for this.

Along with the read_events, fix a test so that it now complies
with this behaviour (and we have this tested)

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>