return only error value to students when codejail raises an exception

remove comment for translators

inst.args[0] is more reliable than inst.message

consistent escaping

common/lib/xmodule/xmodule/capa_base.py

@@ -1232,8 +1232,12 @@ class CapaMixin(ScorableXBlockMixin, CapaFields):
             # Otherwise, display just an error message,
             # without a stack trace
             else:
-                # Translators: {msg} will be replaced with a problem's error message.
-                msg = inst.message
+                escaped_message = cgi.escape(inst.args[0])
+                try:
+                    # only return the error value of the exception
+                    msg = escaped_message.split("\\n")[-2].split(": ", 1)[1]
+                except IndexError:
+                    msg = escaped_message
 
             return {'success': msg}
 

common/lib/xmodule/xmodule/tests/test_capa_module.py

@@ -828,6 +828,42 @@ class CapaModuleTest(unittest.TestCase):
             # Expect that the number of attempts is NOT incremented
             self.assertEqual(module.attempts, 1)
 
+    def test_submit_problem_error_with_codejail_exception(self):
+
+        # Try each exception that capa_module should handle
+        exception_classes = [StudentInputError,
+                             LoncapaProblemError,
+                             ResponseError]
+        for exception_class in exception_classes:
+
+            # Create the module
+            module = CapaFactory.create(attempts=1)
+
+            # Ensure that the user is NOT staff
+            module.system.user_is_staff = False
+
+            # Simulate a codejail exception 'Exception: test error'
+            with patch('capa.capa_problem.LoncapaProblem.grade_answers') as mock_grade:
+                try:
+                    raise ResponseError(
+                        'Couldn\'t execute jailed code: stdout: \'\', '
+                        'stderr: \'Traceback (most recent call last):\\n'
+                        '  File "jailed_code", line 15, in <module>\\n'
+                        '    exec code in g_dict\\n  File "<string>", line 67, in <module>\\n'
+                        '  File "<string>", line 65, in check_func\\n'
+                        'Exception: test error\\n\' with status code: 1',)
+                except ResponseError as err:
+                    mock_grade.side_effect = exception_class(err.message)
+                get_request_dict = {CapaFactory.input_key(): '3.14'}
+                result = module.submit_problem(get_request_dict)
+
+            # Expect an AJAX alert message in 'success' without the text of the stack trace
+            expected_msg = 'test error'
+            self.assertEqual(expected_msg, result['success'])
+
+            # Expect that the number of attempts is NOT incremented
+            self.assertEqual(module.attempts, 1)
+
     def test_submit_problem_other_errors(self):
         """
         Test that errors other than the expected kinds give an appropriate message.