better error message for running harvest without the right environment variables

[thraxil]

currently, if you run "./manage.py harvest" without setting LETTUCE_SKIP_SELENIUM or otherwise having phantomjs/chromedriver/whatever setup, you get confusing, unhelpful messages about imports failing etc. This shouldn't be too hard to detect and give the developer a bit more of a useful clue on what they are missing and how they should fix it.