logging: Log exception name also in auth_complete.

`HTTPError` has empty string for `str(HTTPError())`. Logging it
as it is would not be much helpful. So, this commits adds code
to log the name of error also.

zproject/backends.py

@@ -1362,7 +1362,7 @@ class SocialAuthMixin(ZulipAuthMixin, ExternalAuthMethod):
             # the flow or the IdP is unreliable and returns a bad http response),
             # don't throw a 500, just send them back to the
             # login page and record the event at the info log level.
-            self.logger.info(str(e))
+            self.logger.info(f"{e.__class__.__name__}: {str(e)}")
             return None
         except SocialAuthBaseException as e:
             # Other python-social-auth exceptions are likely