lib: return human-readable error messages to the northbound clients

lib: return human-readable error messages to the northbound clients

Instead of returning only error codes (e.g. NB_ERR_VALIDATION)
to the northbound clients, do better than that and also return
a human-readable error message. This should make FRR more
automation-friendly since operators won't need to dig into system
logs to find out what went wrong in the case of an error.

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