Fixed in r3321. Commit message:

general script infrastructure:
* Fail noisily im tmpdirs are not left empty by the jobs.
* Added some error reporting, separated between regular errors (errors of
  the commands that were called) and critical errors (errors of the driver
  script itself). Critical errors cause immediate termination after the
  currently running jobs.

do_preprocess:
* Translator and preprocessor are now considered to fail if they write
  anything to stderr.

The new translator fails on some inputs (e.g. Pipes-NoTankage-50), and when it
does, the do_preprocess script is happy to reuse the output.sas file left over
from the previous translator run.

As a result, some of the "solved pipesworld tasks" in our latest experiments are
actually from the Mystery domain.

Proposed fix: do_preprocess should delete the inputs in the temp dir after
successful runs so that there is nothing left over in the next run.