Change error message wording on UnicodeDecodeError
Change error message wording on UnicodeDecodeError

As far as I understand the whole unicode/encoding a file cannot be "in unicode" because it just consists of bytes which carry text information saved through a particular encoding table (i.e. UTF-8, etc.). 

I don't want to nitpick but it took me a while to wrap my head around the whole UnicodeDecodeErrors and implicit conversion that I don't want further misinformation to spread. In case I'm mistaken, please correct me and just close this pull request.
django-import-export/django-import-export,rhunwicks/django-import-export,django-import-export/django-import-export,PetrDlouhy/django-import-export,daniell/django-import-export,pajod/django-import-export,rhunwicks/django-import-export,jnns/django-import-export,manelclos/django-import-export,PetrDlouhy/django-import-export,bmihelac/django-import-export,jnns/django-import-export,copperleaftech/django-import-export,daniell/django-import-export,daniell/django-import-export,pajod/django-import-export,bmihelac/django-import-export,pajod/django-import-export,manelclos/django-import-export,brillgen/django-import-export,bmihelac/django-import-export,copperleaftech/django-import-export,bmihelac/django-import-export,django-import-export/django-import-export,django-import-export/django-import-export,rhunwicks/django-import-export,daniell/django-import-export,brillgen/django-import-export,jnns/django-import-export,PetrDlouhy/django-import-export,manelclos/django-import-export,brillgen/django-import-export,copperleaftech/django-import-export,jnns/django-import-export,copperleaftech/django-import-export,pajod/django-import-export,PetrDlouhy/django-import-export,brillgen/django-import-export