Add thread list endpoint to the new discussion API

This is an initial implementation that only allows retrieval of all
threads for a course and only returns an easily computed subset of the
fields that are needed, in order to keep this change from getting too
large.

JIRA: MA-641

openedx/core/lib/api/view_utils.py

@@ -1,6 +1,7 @@
 """
 Utilities related to API views
 """
+from django.core.exceptions import NON_FIELD_ERRORS, ValidationError
 from django.http import Http404
 
 from rest_framework.exceptions import APIException
@@ -19,10 +20,31 @@ class DeveloperErrorViewMixin(object):
         """
         return Response({"developer_message": developer_message}, status=status_code)
 
+    def make_validation_error_response(self, validation_error):
+        """
+        Build a 400 error response from the given ValidationError
+        """
+        if hasattr(validation_error, "message_dict"):
+            response_obj = {}
+            message_dict = dict(validation_error.message_dict)
+            non_field_error_list = message_dict.pop(NON_FIELD_ERRORS, None)
+            if non_field_error_list:
+                response_obj["developer_message"] = non_field_error_list[0]
+            if message_dict:
+                response_obj["field_errors"] = {
+                    field: message_dict[field][0]
+                    for field in message_dict
+                }
+            return Response(response_obj, status=400)
+        else:
+            return self.make_error_response(400, validation_error.messages[0])
+
     def handle_exception(self, exc):
         if isinstance(exc, APIException):
             return self.make_error_response(exc.status_code, exc.detail)
         elif isinstance(exc, Http404):
             return self.make_error_response(404, "Not found.")
+        elif isinstance(exc, ValidationError):
+            return self.make_validation_error_response(exc)
         else:
             raise