Trigger AttributeError in ValidationError.message_dict when error_dict is missing.

The goal of this change is twofold; firstly, matching the behavior of Django 1.6
and secondly, an AttributeError is more informative than an obscure ValueError
about mismatching sequence lengths.

Refs #20867.
Merge pull request #2048 from loic/ValidationError.message_dict

Trigger AttributeError in ValidationError.message_dict when error_dict is missing.
Made flake8 ignore the .git directory.
Merge pull request #2052 from loic/setup.cfg

Made flake8 ignore the .git directory.
(Re-)added GeoDjango instructions for building pysqlite2 correctly.

This is a partial undo of 1b142ef5dd.
Updated a bunch of hyperlinks in documentation
Removed an erroneous leading slash introduced by a626bdf648a.