In Python 2, __repr__ and __str__ shouldn't return unicode.
This caused a problem for example when logging-debug a filter on Thumbnail.url,
as the logger couldn't encode the unicode repr.

Signed-off-by: Simon Lipp <laiquo@hwold.net>

flake8 --select W391,E302,E301,E304

autopep8 can't fix W391 even though it claims it can.
Fixed using a simple custom script.

Remove useless trailing \
Remove trailing spaces
Add missing empty lines

autopep8 -ir -j2 --select=E301,E302,E502,W291,W293,W391 .

Diff quickly checked.

Modules can raise this exception when they want to print errors to user