[ctu] Don't silently ignore exceptions thrown in pre-analysis

[Szelethus]

Exceptions thrown in a thread pool by default stay in the thread pool (won't be re-raised at the call to map_async or similar functions). However, you can query whether an exception occured with the help of AsyncResult.successful().

This patch is simple -- don't pretend that pre-analysis exceptions can be ignored, just because they run asyncronously, exceptions there are just as severe as elsewhere.