Rescue NotIdentifiedByImageMagickError for Paperclip versions < 3.0.0.

The exception class `Paperclip::Errors::NotIdentifiedByImageMagickError` was introduced in Paperclip 3.0.0; in earlier versions it was `Paperclip::NotIdentifiedByImageMagickError`. 

This commit checks the Paperclip version and rescues the corresponding exception class.