Update the Active Model deprecation warning

This message more clearly communicates how to access the attribute and message keys that you would expect to get when using the previous API.

before you might iterate over errors like,

errors.each do |attribute, message|
  # My error code here
end

This message helps the user find the methods on error that match the previous API.

activemodel/lib/active_model/errors.rb

@@ -230,7 +230,8 @@ def each(&block)
           parameter like this:
 
           person.errors.each do |error|
-            error.full_message
+            attribute = error.attribute
+            message = error.message
           end
 
           You are passing a block expecting two parameters,