Clear caught exception after it's raised

The problem is that when we have 2 tests running and exception is thrown in test 1, then test 2 will fail due to the following reasons:

1. `Watir::Rails.error` is not cleared, so the next run checker will raise it again.
2. Middleware don't raise exception, so server (thin in my case) will stop. We need to re-raise it in middleware.

This pull request addresses this issue.