made logging message more useful, less crashy

using the bare instance as an argument to the string formating, which
usually calls __unicode__, which in all likelihood tries to access a field
from the translated model, ending up with a

    type object 'NoTranslation' has no attribute 'foobar'

error.
Merge pull request #100 from piquadrat/feature/dont-crash-while-logging

made logging message more useful, less crashy