net: lwm2m: Suppress many LOG_ERR() that are not fatal

Suppress many LOG_ERR() messages from LwM2M registry
that are not necessary runtime errors.

Libraries and applications should be able to do following
without causing LOG_ERR to be produced:

* Checking existence of object, resource or resource instance
  using lwm2m_engine_get_res(), lwm2m_engine_get_res_inst()
  or path_to_objs(). These are only exposed in internal header.
* Delete object instance or resource instance without checking
  if it exits.

As there is no public API to check existence of some path,
application is much easier to write in a way that it directly
calls just lwm2m_get...(), lwm2m_set...(), lwm2m_delete...()
and trust the return code of -ENOENT.

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