CustomResponse: fix log message.

common/lib/capa/capa/responsetypes.py

@@ -1521,17 +1521,18 @@ class CustomResponse(LoncapaResponse):
             # ordered list of answers
             submission = [student_answers[k] for k in idset]
         except Exception as err:
-            msg = _(
-                "[courseware.capa.responsetypes.customresponse] error getting"
-                " student answer from {student_answers}"
-                "\n idset = {idset}, error = {err}"
-            ).format(
-                student_answers=student_answers,
+            msg = u"[courseware.capa.responsetypes.customresponse] {message}\n idset = {idset}, error = {err}".format(
+                message= _("error getting student answer from {student_answers}").format(student_answers=student_answers),
                 idset=idset,
                 err=err
-            );
+            )
 
-            log.error(msg)
+            log.error(
+                "[courseware.capa.responsetypes.customresponse] error getting"
+                " student answer from %s"
+                "\n idset = %s, error = %s",
+                student_answers, idset, err
+            )
             raise Exception(msg)
 
         # global variable in context which holds the Presentation MathML from dynamic math input