Add ClosedError message to the initializer

actionpack/lib/action_dispatch.rb

@@ -60,6 +60,7 @@ module ActionDispatch
     autoload :Static
   end
 
+  autoload :ClosedError, 'action_dispatch/middleware/closed_error'
   autoload :MiddlewareStack, 'action_dispatch/middleware/stack'
   autoload :Routing
 

actionpack/lib/action_dispatch/middleware/closed_error.rb

@@ -0,0 +1,7 @@
+module ActionDispatch
+  class ClosedError < StandardError #:nodoc:
+    def initialize(kind)
+      super "Cannot modify #{kind} because it was closed. This means it was already streamed back to the client or converted to HTTP headers."
+    end
+  end
+end

actionpack/lib/action_dispatch/middleware/flash.rb

@@ -51,7 +51,7 @@ module ActionDispatch
       def close!; @closed = true end
 
       def []=(k, v)
-        raise ClosedError, "Cannot modify flash because it was closed. This means it was already streamed back to the client or converted to HTTP headers." if closed?
+        raise ClosedError, :flash if closed?
         @flash[k] = v
         @flash.discard(k)
         v
@@ -84,7 +84,7 @@ module ActionDispatch
       def close!; @closed = true end
 
       def []=(k, v) #:nodoc:
-        raise ClosedError, "Cannot modify flash because it was closed. This means it was already streamed back to the client or converted to HTTP headers." if closed?
+        raise ClosedError, :flash if closed?
         keep(k)
         super
       end
@@ -208,7 +208,4 @@ module ActionDispatch
       end
     end
   end
-
-  class ClosedError < StandardError #:nodoc:
-  end
 end