Description of problem:

We should have better error handling in this case while deleting subnet that connected to instance

Error text:

Expected(204) <=> Actual(409 Conflict) excon.error.response :body => "{\"NeutronError\": {\"message\": \"Unable to complete operation on subnet 2e9fbbe0-6bb3-4849-a981-d0a3406a45dc: One or more ports have an IP allocation from this subnet.\", \"type\": \"SubnetInUse\", \"detail\": \"\"}}" :cookies => [ ] :headers => { "Content-Length" => "204" "Content-Type" => "application/json" "Date" => "Tue, 22 Nov 2016 15:13:05 GMT" "X-Openstack-Request-Id" => "req-bdccc09b-4693-41e9-8be7-3ef3ec4457d6" } :host => "10.0.0.104" :local_address => "10.0.0.36" :local_port => 44258 :path => "/v2.0/subnets/2e9fbbe0-6bb3-4849-a981-d0a3406a45dc" :port => 9696 :reason_phrase => "Conflict" :remote_ip => "10.0.0.104" :status => 409 :status_line => "HTTP/1.1 409 Conflict\r\n" [cloud_subnet/button] 

Version-Release number of selected component (if applicable):

5.7.0.11

How reproducible:
100%

Steps to Reproduce:
1. Navigate to: Networks --> Subnets
2. Tick a subnet connected to an instance
3. Click Configuration --> Delete selected Cloud Subnets.

Actual results:

Generic error page

Expected results:

A comprehensive message that deleting a cloud subnet attached to instance is not allowed

Additional info: