vpp-api: Add context to VPPStatsIOError.

* Provide context information and return meaningful information.
VPPStatsIOError: Stat segment client connection returned: -2 Stat client socket open but couldn't connect.

Change-Id: I14ea35c58452a2cb7598a7efd136f0c7e1f5743a
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>