Fix unary operator expected

Added in a way to easily inspect just the output of the command under the scope to more easily catch things like this

```
cucumber features/git-prune-branches.feature OUTPUT_COMMANDS=1
```

Tacked on a couple more commits fixing bugs founds using OUTPUT_COMMANDS
* Git extract had some similar errors
* Abort / continue checks weren't working properly (git extract was listing a continue message and trying to remove a non existent continue file)
I'm not sure I get the point of the `run_and_store` method. It seems to do almost the same as the `run` method. If we refactor `run` to always store the result in `@last_run_result`, and consider `ENV['OUTPUT_COMMANDS']`, can we get rid of `run_and_store`?