Adds missing expected_errors for V3 API multinic extension

The V3 API multinic extension is missing the expected errors
decorators which means that unexpected internal Nova exceptions
are not caught correctly. This patch adds the expected error
decorators.

Change-Id: I99311275197bb5123079c8910855386252028dec
Closes-Bug: 1303781
Merge "Adds missing expected_errors for V3 API multinic extension"