*: convert northbound callbacks to new error handling model

*: convert northbound callbacks to new error handling model

The northbound configuration callbacks should now print error
messages to the provided buffer (args->errmsg) instead of logging
them directly.  This will allow the northbound layer to forward the
error messages to the northbound clients in addition to logging them.

NOTE: many callbacks are returning errors without providing any
error message. This needs to be fixed long term.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>