Add an explicit exception is PIL/pillow is missing.

I'm not sure if this really needs a fix, since pillow is a requirement.  But an explicit error message is
better than having error messages like `AttributeError: 'NoneType' object has no attribute 'open'`

https://groups.google.com/d/msg/nikola-discuss/MYysrF1-Anc/5jm_KU9ZchYJ