vpp_papi: Add custom exceptions.

vpp_papi:  Add custom exceptions.

This patchset adds and raises the following custom exception classes:

* class VPPApiError(Exception):
* class VPPNotImplementedError(NotImplementedError):
* class VPPIOError(IOError):
* class VPPRuntimeError(RuntimeError):
* class VPPValueError(ValueError):

* class VPPSerializerValueError(ValueError):

* class VPPStatsIOError(IOError):
* class VPPStatsClientLoadError(RuntimeError):

* class VppTransportShmemIOError(IOError):

* class VppTransportSocketIOError(IOError)

Change-Id: Ia40900fd2dcef148d01125d6c691329fc666901e
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>