change test exception handling to actually work

exceptions occurring inside the test framework when in parallel aren't handled properly, and cause the framework to hang.
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- [ ] <a href='#crh-comment-Pull 236d657feeb64ac994d2df344ec17bfa884e0133 lettuce/__init__.py 7'></a> <img src='http://www.codereviewhub.com/site/github-remaining.png' height=16 width=60>&nbsp;<b><a href='https://github.com/Spentry/tmwy-lettuce-parallel/pull/8#discussion_r20538095'>File: lettuce/__init__.py:L222-230</a></b>
- <a href='https://github.com/conslo'><img border=0 src='https://avatars.githubusercontent.com/u/450860?v=3' height=16 width=16'></a> this is a problem


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:+1: