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.

• python-social-auth/social/backends/facebook.py

  Lines 62 to 63 in f13a5cc

  	raise AuthCanceled(self, data.get('error_message') or
  	data.get('error_code'))

• python-social-auth/social/backends/oauth.py

  Line 171 in f13a5cc

  raise AuthCanceled(self, 'User refused the access')

• python-social-auth/social/backends/oauth.py

  Line 376 in f13a5cc

  raise AuthCanceled(self, data.get('error_description', ''))

• python-social-auth/social/backends/oauth.py

  Line 380 in f13a5cc

  raise AuthCanceled(self, data['denied'])

[omab]

I've ported this PR into social-core

Thanks!