Add referer to "Spider error processing" log message

\+ fixed some pep8 issues

Motivation: I want to be able to see response request referer in cases like this one:

![selection_009](https://cloud.githubusercontent.com/assets/744331/3552986/959b8d0e-08fe-11e4-89f6-13b0806ad28c.jpeg)

Also I wonder if messages like "Spider error processing", "Error downloading" etc. should be handled by logformatter - to make it possible to override/disable them.