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?

@arthurnn it used to raise in an `else` block

but it looks like load-errors happen whenever a fixture is loaded for a `habtm` relation that is not a `has_many :through`, so does not have a class associated. because of this some rails tests kept failing. removing the else block seemed reasonable to keep backwards-compatibility.