discover/udev: don't propagate non-fatal errors from udev_process

Currently, we have a bug where non-zero return codes from
udev_handle_dev_* cause the udev worker from deregistering from the
waiter poll loop. This is becasue udev_process is propagating these
errors, causing the deregistration.

This change stops propagation of non-fatal errors, so we don't
deregister.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>