Return proper error message when network conflicts

Fixes bug 1158075
When creating a network which is already existing, we got
an internal server error which can't show the error reason.
This patch returns conflict error(HTTP 409) instead of
internal server error(HTTP 500), so that users can know
the error reason from the error message.

Change-Id: I9e39a7152e37756b34b1c8d77de057012743f0f7
Merge "Return proper error message when network conflicts"