net: lwm2m: Fix Registration failue reporting

In case LwM2M server or bootstrap server rejected
Registration/Registration Update/Deregsitration attempt, there were no
reasonable notification to the application. Fix this by reporting
LWM2M_RD_CLIENT_EVENT_*_FAILURE in such case.

Addtitionaly, remove pointless ENGINE_DEREGISTER_FAILED event, which
have no use in the state machine.

Finally, simplify the response code logging to prevent code duplication.

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