NVP plugin: return 409 if wrong router interface info on remove

Bug 1152167

Raise an exception causing a 409 error to be returned if port_id or
subnet_id specified in request body do not exist before further
processing the operation (which will cause a 500 error to be
returned to the user)

Change-Id: I347dd86da87ce5b1e57e139e4c21d4533a456cbc
Merge "NVP plugin: return 409 if wrong router interface info on remove"