net: lwm2m: Handle socket errors

So far socket errors reported by poll/recvfrom were ignored, which could
lead to an unexpected behavior when socket was left in an undefined
state.

Fix this, by requesting a re-registration in the LWM2M state machine,
which will close the faulty socket and open a new one. Note, that simply
closing and re-opening a socket in the lwm2m engine would not work,
since this would silently invalidate any open observations on the
lwm2m server side (due to port number change). Triggering a fresh
registration will notify the server to update its observations.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>