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?

We're changing the signature of a public method here. If that's okay, it should probably be `:nodoc:`.