Add YangNetconfError
This is a yang-data-api specialization of a well-known RFC4741/6020
construct, which we have tried to model in yang.common.RpcError and
then via yang.common.YangError.
Unlike YangError, which is directly glued to exceptions,
YangNetconfError has a builder-based immutable implementation, which can
be attached to other objects via YangNetconfErrorAware.
JIRA: YANGTOOLS-1305
Change-Id: I7aa1b587ea74c552996b62514861b4b8b262361e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>