Allow error messages with non-ascii characters to be handled correctly

Also, add a test for this behavior.

CHANGELOG.rst

@@ -42,6 +42,8 @@ setting now run entirely outside the Python sandbox.
 
 Blades: Added tests for Video Alpha player.
 
+Common: Have the capa module handle unicode better (especially errors)
+
 Blades: Video Alpha bug fix for speed changing to 1.0 in Firefox.
 
 Blades: Additional event tracking added to Video Alpha: fullscreen switch, show/hide

common/lib/xmodule/xmodule/capa_module.py

@@ -781,7 +781,7 @@ class CapaModule(CapaFields, XModule):
             # Otherwise, display just an error message,
             # without a stack trace
             else:
-                msg = "Error: %s" % str(inst.message)
+                msg = u"Error: {msg}".format(msg=inst.message)
 
             return {'success': msg}
 

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

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 """Tests of the Capa XModule"""
 #pylint: disable=C0111
 #pylint: disable=R0904
@@ -520,6 +521,33 @@ class CapaModuleTest(unittest.TestCase):
             # Expect that the number of attempts is NOT incremented
             self.assertEqual(module.attempts, 1)
 
+    def test_check_problem_error_nonascii(self):
+
+        # Try each exception that capa_module should handle
+        for exception_class in [StudentInputError,
+                                LoncapaProblemError,
+                                ResponseError]:
+
+            # Create the module
+            module = CapaFactory.create(attempts=1)
+
+            # Ensure that the user is NOT staff
+            module.system.user_is_staff = False
+
+            # Simulate answering a problem that raises the exception
+            with patch('capa.capa_problem.LoncapaProblem.grade_answers') as mock_grade:
+                mock_grade.side_effect = exception_class(u"ȧƈƈḗƞŧḗḓ ŧḗẋŧ ƒǿř ŧḗşŧīƞɠ")
+
+                get_request_dict = {CapaFactory.input_key(): '3.14'}
+                result = module.check_problem(get_request_dict)
+
+            # Expect an AJAX alert message in 'success'
+            expected_msg = u'Error: ȧƈƈḗƞŧḗḓ ŧḗẋŧ ƒǿř ŧḗşŧīƞɠ'
+            self.assertEqual(expected_msg, result['success'])
+
+            # Expect that the number of attempts is NOT incremented
+            self.assertEqual(module.attempts, 1)
+
     def test_check_problem_error_with_staff_user(self):
 
         # Try each exception that capa module should handle