Fix to throw correct error code for bad attribute

Currently the neutron network API throws up error code 500 for
the extended attribute for segmentation id. This can be reproduced
if the user types in a random string in place of an integer value
for the segmentation id. The proper behavior should throw an error
code 400 with the appropriate failure message. This patch fixes the
same issue and covers it with a test case.

Change-Id: I4735e20f5b8b23c5b2a9d896415c2e84561a279c
Closes-bug: #1348056
Merge "Fix to throw correct error code for bad attribute"