Catch exception with duplicate username/email

Previously, there was no catch for the AccountValidationError
exception raised by the account creation function. If, for some
reason, the user made it past the first check for a duplicate
username/email, then the exception was raised, uncaught, and
crashed the server.

openedx/core/djangoapps/user_api/views.py

@@ -25,7 +25,7 @@ from openedx.core.lib.api.authentication import SessionAuthenticationAllowInacti
 from openedx.core.lib.api.permissions import ApiKeyHeaderPermission
 from student.cookies import set_logged_in_cookies
 from student.forms import get_registration_extension_form
-from student.views import create_account_with_params
+from student.views import create_account_with_params, AccountValidationError
 from util.json_request import JsonResponse
 
 from .accounts import (
@@ -364,6 +364,9 @@ class RegistrationView(APIView):
 
         try:
             user = create_account_with_params(request, data)
+        except AccountValidationError as err:
+            errors = { err.field: [{"user_message": err.message}] }
+            return JsonResponse(errors, status=409)
         except ValidationError as err:
             # Should only get non-field errors from this function
             assert NON_FIELD_ERRORS not in err.message_dict