remote_user_sso: Improve invalid email message.

Show a user friendly message to the user if email is invalid.
Currently we show a generic message:
"Your username or password is incorrect."

zerver/tests/test_auth_backends.py

@@ -1309,6 +1309,13 @@ class TestZulipRemoteUserBackend(ZulipTestCase):
             self.assertEqual(result.status_code, 302)
             self.assertIs(get_session_dict_user(self.client.session), None)
 
+    def test_login_failure_due_to_invalid_email(self):
+        # type: () -> None
+        email = 'hamlet'
+        with self.settings(AUTHENTICATION_BACKENDS=('zproject.backends.ZulipRemoteUserBackend',)):
+            result = self.client_post('/accounts/login/sso/', REMOTE_USER=email)
+            self.assert_json_error_contains(result, "Enter a valid email address.", 400)
+
     def test_login_failure_due_to_missing_field(self):
         # type: () -> None
         with self.settings(AUTHENTICATION_BACKENDS=('zproject.backends.ZulipRemoteUserBackend',)):

zerver/views/auth.py

@@ -106,6 +106,12 @@ def remote_user_sso(request):
     except KeyError:
         raise JsonableError(_("No REMOTE_USER set."))
 
+    # Django invokes authenticate methods by matching arguments, and this
+    # authentication flow will not invoke LDAP authentication because of
+    # this condition of Django so no need to check if LDAP backend is
+    # enabled.
+    validate_login_email(remote_user_to_email(remote_user))
+
     user_profile = authenticate(remote_user=remote_user, realm_subdomain=get_subdomain(request))
     return login_or_register_remote_user(request, remote_user, user_profile)