net: lwm2m: Allow to return error codes by content encoders/decoders

Rework the content encoder/decoder API to allow to return negative
values as well. This allows a proper error reporting and error handling,
as it's now possible to differentiate when there is no content to write
(retuned 0) or and error occured, and further processing should be
aborted.

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