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?

@matthewd this whole message thing is kinda wishy washi because it tries to load something that might just not be there. fixtures for `has_and_belongs_to_many` join tables are a good example because they don't have a class associated, so there probably is a load-error all the time.