Message on AR::UnknownAttributeError should include the class name of a record

This would be helpful if 2 models have an attribute that has a similar name to the other. e.g.
before:

```ruby
User.new(name: "Yuki Nishijima", projects_attributes: [name: "kaminari"])
# => ActiveRecord::UnknownAttributeError: unknown attribute: name
```

after:

```ruby
User.new(name: "Yuki Nishijima", projects_attributes: [name: "kaminari"])
# => ActiveRecord::UnknownAttributeError: unknown attribute on User: name
```