As suggested by Renato, add error codes that are specific
to the various phases of a northbound callback. These can
be used by the daemons when logging an error. The reasoning
is that validation errors typically mean that there is an
inconsistency in the configuration, a prepare error means
that we are running out of resources, and abort/apply errors
are bugs that need to be reported to the devs.
Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>