do not call `Apipie::Extractor.call_finished` when exception raised

When I record example from my controller spec,  and the spec failed for some reason, I will see a stack trace like this:

```ruby
NoMethodError:
       undefined method `each' for nil:NilClass
     # ./spec/controllers/examples_controller_spec.rb:33:in `block (4 levels) in <top (required)>'
     # ./spec/controllers/examples_controller_spec.rb:39:in `block (4 levels) in <top (required)>'
```
it take me nearly one hour to figure what's actually wrong.

actually, the error was raised from : `lib/apipie/extractor/collector.rb:56`, because `apipie` can not record some data when spec failed.

I think  it dosen't make sense to call the `Apipie::Extractor.call_finished` when spec already failed, so I create this pull request.

thoughts?