net: lwm2m: send REGISTRATION_TIMEOUT event on error

When socket errors call sm_handle_timeout_state() we might be
in a state where application assumes we are in registered state
but we are dropping it.
Therefore we must ensure that all registration states emit either
REGISTRATION_TIMEOUT event for application to indicate that
we have lost the connection to server.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>