Better diagnostics when communication with subprocess fails

Fixes issue #5.

Now I can see that when tox tries to run zope-testrunner -j99, the
subprocess fails with

  /full/path/to/.tox/py27/bin/python: can't open file '.tox/py27/bin/zope-testrunner': [Errno 2] No such file or directory

It remains merely to fix _that_ error.