Fix error while creating l2 gateway services in nvp

Ensure that Conflict errors are not masked; also ensure
that coding errors do not mask NVP API errors.

Fixes bug #1244259

Change-Id: I27586d72af89fd39f6443dbab33f0e4e762b0c99
(cherry picked from commit 94950eeb22c95da6b2115ea99f6131700decaa09)
Merge "Fix error while creating l2 gateway services in nvp" into stable/havana