yang-common: allow application-specific RpcErrors

yang.common.RpcError does not model https://tools.ietf.org/html/rfc6241#section-4.3 correctly, specifically it misinterprets error-info as a String field, whereas the RFC defines it as a tag, which can be extended with richer content (https://tools.ietf.org/html/rfc6241#appendix-A, see ' error-tag: missing-attribute').

At least the cases covered in RFC6241 need to be covered, but there seems to be appetite to add more: https://lists.opendaylight.org/pipermail/yangtools-dev/2017-January/001730.html.

We need to devise a way to channel this information in a way which will allow both XML and JSON encodings to work correctly. One way of achieving that would we to allow getInfo() to return a NormalizedNode structure, which would be allowed to only contain String leaves – but that is not possible due to this class being shared by both BI and BA applications. We may need to move RpcResult to its proper domain, i.e. have it defined in md-sal for both BA and BI interfaces and perform transcoding.