Provide human readable error description

[mauriciovasquezbernal]

The error message that is returned when the client performs a non-valid request does not contain a description of the error.

For instance:

$ polycubectl hw0 ports add p1 perar=dasda
{"ietf-restconf:errors":{"error":[{"error-type":"application","error-tag":"perar"}]}}

It would be nice to provide a human readable error description in those cases. I think we can use the error-message field of http://www.netconfcentral.org/modules/ietf-restconf#error.146.