GUI issue while trying to clear gateway if RouterExternalGatewayInUseByFloatingIp

[Sunder Jadam]

Detail Description :
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While trying to clear gateway when there were any associates IP for the router. Instead of throuing the error message on dashboard dashboard is coming out with error message "Something went wrong!

An unexpected error has occurred. Try refreshing the page. If that doesn't help, contact your local administrator."

Steps to reproduce issue:
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Create linux router, set gateway and associate IP to VM. so that we can reach to external world.
now try to remove gateway by clicking clear gateway under router tab.
This should give the error message saying "RouterExternalGatewayInUseByFloatingIp: Gateway cannot be updated for router <>, since a gateway to external network <> is required by one or more floating IPs."

but at step # 3, The horizon is coming out with "Something went wrong!

An unexpected error has occurred. Try refreshing the page. If that doesn't help, contact your local administrator."

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/quantum/api/v2/resource.py", line 82, in resource
    result = method(request=request, **args)
  File "/usr/lib/python2.7/dist-packages/quantum/api/v2/base.py", line 449, in update
    obj = obj_updater(request.context, id, **kwargs)
  File "/usr/lib/python2.7/dist-packages/quantum/db/extraroute_db.py", line 60, in update_router
    context, id, router)
  File "/usr/lib/python2.7/dist-packages/quantum/db/l3_db.py", line 165, in update_router
    self._update_router_gw_info(context, id, gw_info)
  File "/usr/lib/python2.7/dist-packages/quantum/db/l3_db.py", line 196, in _update_router_gw_info
    router_id=router_id, net_id=gw_port['network_id'])
RouterExternalGatewayInUseByFloatingIp: Gateway cannot be updated for router ef876a88-1b86-4ebf-af0e-adadff22e02b, since a gateway to external network 10ef4709-7e05-4c2e-b9de-5b0578573d7d is required by one or more floating IPs.

Impact : No operational impact but from end user point of view "GUI crash has been crashed".
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