Include additional data in ydk exception

[ghost]

Related to #23.

There are multiple instances where xml output returned by the router is in-correct. Consequently, ydk returns 'YPYModelError: 0 already in list'. In such instances, it will be good to get the xml output in raw format.

It may be a valid approach to include extra data (like invalid XML) in the python exception. see for example: http://legacy.python.org/dev/peps/pep-0473/. Maybe we should reopen this issue on the ydk-gen repo?

[ghost]

The error raised when doing crud.read on faulty XML now includes the faulty XML payload. This can be used by code similar to below

try:
    res = crud.read(provider, obj)
catch YPYModelError as err:
   print(err.msg, err.payload)