Raises the correct exception when adding interface to subnet

This fix changes the exception raised when trying to
add a router interface on a subnet while the
router already has a interface on that subnet.

Change-Id: Ic6034f1c1badf21ff6fafac0441de43857fd0e9f
(cherry picked from commit 47eb00b9b09173ff573df41b5e3280b52e1856f1)
Merge "Raises the correct exception when adding interface to subnet" into R1.10