error: Fix ProcessingError message and logging

error: Fix ProcessingError message and logging

We can't check if message is None if message wasn't an argument to
__init__().  Also:

* import sys for sys.version
* explicitly format strings passed to _LOG.warning(), otherwise you'll
  get the following:

    >>> LOG.warning('abc', 'def')
    Traceback (most recent call last):
      ...
    TypeError: not all arguments converted during string formatting

Signed-off-by: W. Trevor King <wking@tremily.us>