Cleanup aggregate_generated_bindings.py

Now that aggregate_generated_bindings.py is editable,
I've added a link to the design doc,
fixed an incorrect use of old-style string exceptions,
and cleaned up the return slightly.

TBR=haraken

Review URL: https://codereview.chromium.org/189513013

git-svn-id: svn://svn.chromium.org/blink/trunk@168822 bbb929c8-8fbe-4397-9dbb-9b2b20218538