Don't lose custom exception message on raising AuthCanceled

[dotsbb]

There are 4 usages of AuthCanceled with custom error message. In all those cases the message will be lost without this fix.

• https://github.com/omab/python-social-auth/blob/f13a5cc3c8670a1a3bbc16410bb30ac0423d9934/social/backends/facebook.py#L62-L63
• https://github.com/omab/python-social-auth/blob/f13a5cc3c8670a1a3bbc16410bb30ac0423d9934/social/backends/oauth.py#L171
• https://github.com/omab/python-social-auth/blob/f13a5cc3c8670a1a3bbc16410bb30ac0423d9934/social/backends/oauth.py#L376
• https://github.com/omab/python-social-auth/blob/f13a5cc3c8670a1a3bbc16410bb30ac0423d9934/social/backends/oauth.py#L380

[omab]

I've ported this PR into social-core

Thanks!