don't log errors when all is fine

this change prevents rails from logging verbose stack-traces when the exceptional case has already been handled by the developers.

so when your config looks like this

```ruby
class ActiveSupport::TestCase
  set_fixture_class fixture_class_name: SomeModel, [...]
  fixtures :all
  [...]
```

it does not print a warning containing a possible 100 lines long stack-trace.

the problem only occurs on weird edge-cases where models are inherited+inlined in other model-classes. so something like this

```ruby
class User < ActiveRecord::Base
  class Admin  < User
  end
end
```

what would be a good way to test this?

This message needs rephrasing. Firstly, because of ordering, it implies that if they haven't added it, that's definitely going to be the cause. Secondly, it's lying by omission: "make sure you added it"? We *know* it's not there: we just checked.