Sometimes, exceptions thrown by modules during execute_module() are NOT trapped

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This is a generalisation of issue 98: Today (20100210) I saw this behaviour 
with a DICOMReader and a DICOM series that can't be sorted.  The reader 
module raised the error (which I traced with pdb) that got back to the 
scheduler, but somehow no trap.

I am as of yet not able to reproduce.

Original issue reported on code.google.com by cpbotha on 10 Feb 2010 at 12:20