Detect when clamscand returns an error

Previously if the clamscand command failed - eg due to the clam daemon
not running - the wrapper would return that a virus was detected. This
led to very misleading test failures if clam was broken.
PEP8 our changes from previous commit
Merge pull request #197 from alphagov/clamav-errors

fixing scanned_file.py to handle clamdscan errors