update error message to be more specific to what the user needs to fix

cms/djangoapps/contentstore/tests/test_contentstore.py

@@ -990,7 +990,7 @@ class ContentStoreTest(ModuleStoreTestCase):
     def test_create_course_duplicate_course(self):
         """Test new course creation - error path"""
         self.client.post(reverse('create_new_course'), self.course_data)
-        self.assert_course_creation_failed('There is already a course defined with the same organization, course number, and course run. Please change at least one field to be unique.')
+        self.assert_course_creation_failed('There is already a course defined with the same organization, course number, and course run. Please change either organization or course number to be unique.')
 
     def assert_course_creation_failed(self, error_message):
         """

cms/djangoapps/contentstore/views/course.py

@@ -120,7 +120,7 @@ def create_new_course(request):
     if existing_course is not None:
         return JsonResponse(
             {
-                'ErrMsg': _('There is already a course defined with the same organization, course number, and course run. Please change at least one field to be unique.'),
+                'ErrMsg': _('There is already a course defined with the same organization, course number, and course run. Please change either organization or course number to be unique.'),
                 'OrgErrMsg': _('Either of organization or course number must be unique.'),
                 'CourseErrMsg': _('Either of organization or course number must be unique.'),
             }