[rackspace] fix model notfound handling

lib/fog/rackspace/models/servers/flavors.rb

@@ -29,7 +29,7 @@ module Fog
         def get(flavor_id)
           data = connection.get_flavor_details(flavor_id).body['flavor']
           new(data)
-        rescue Excon::Errors::NotFound
+        rescue Fog::Rackspace::Servers::NotFound
           nil
         end
 

lib/fog/rackspace/models/servers/images.rb

@@ -38,7 +38,7 @@ module Fog
         def get(image_id)
           data = connection.get_image_details(image_id).body['image']
           new(data)
-        rescue Excon::Errors::NotFound
+        rescue Fog::Rackspace::Servers::NotFound
           nil
         end
 

lib/fog/rackspace/models/servers/servers.rb

@@ -30,7 +30,7 @@ module Fog
           if server = connection.get_server_details(server_id).body['server']
             new(server)
           end
-        rescue Excon::Errors::NotFound
+        rescue Fog::Rackspace::Servers::NotFound
           nil
         end