A solution to the StackExchange account vs profile problem.

The previous pull request was bad. This one should be good. To repeat what I said in the previous pull request:

I have switched over to your implementation of StackExchange OAuth 2.0, and added my solution to the account vs profile problem.

The StackExchangeOAuth2Adapter.complete_login method now raises OAuth2Error if
the user does not have a StackOverflow profile. The OAuth2LoginView.dispatch
or OAuth2CallbackView.dispatch method catches it and displays a message in the
socialaccount/authentication_error.html template, telling the user why the
authentication failed.