docs(django): document SOCIAL_AUTH_ERROR_TRANSPORT setting

[peterfarrell]

Document the configurable error transport mechanism in SocialAuthExceptionMiddleware to support SPAs and hybrid applications:

• Add documentation for the ErrorTransport enum ('messages', 'query')
• Document configuring single or multiple transport modes
• Document query parameter customization via settings and subclassing
• Document backend-specific transport and parameter overrides
• Document fallback behavior and URL fragment handling

Relates to python-social-auth/social-app-django#1090
Resolves python-social-auth/social-app-django#1067

[peterfarrell]

@nijel woot woot!