Add YangError replacements

yang.common.YangError is a nice first approximation to what we need to communicate, but unfortunately it is not enough as it needs a tie-in to yang-data-api.

Introduce YangNetconfError as an interface similar to YangError and YangNetconfErrorAware, which allows using errors as attachements to exceptions.

Deprecate YangError, so we have clear direction of evolution and there is no ambiguitiy as to which interfaces are preferred.