Add exception handle for External Connectivity

The current ODL/GBP mapping driver does not support external connectivity.
Add exception handling in ODL GBP mapping driver.

Change-Id: I9ce1d752cd8587fd3d423bb6fbf305600072b102
Signed-off-by: Yapeng Wu <yapengwu@gmail.com>
Author: Yapeng Wu <yapengwu@gmail.com>
Co-Authored-By: Yi Yang <yyos1999@gmail.com>