wrap filters/conditionals in a try-except block

allows accessing undefined fields without exception or locals().get(…)
but hides mistakes/typos/etc by evaluating to False without feedback

performance loss compared to the previous version without try-except
is negligible (~20ns for me)

test/test_util.py

@@ -134,19 +134,18 @@ class TestPredicate(unittest.TestCase):
         with self.assertRaises(SyntaxError):
             util.FilterPredicate("(")
 
-        with self.assertRaises(exception.FilterError):
-            util.FilterPredicate("a > 1")(url, {"a": None})
-
-        with self.assertRaises(exception.FilterError):
-            util.FilterPredicate("b > 1")(url, {"a": 2})
+        self.assertFalse(
+            util.FilterPredicate("a > 1")(url, {"a": None}))
+        self.assertFalse(
+            util.FilterPredicate("b > 1")(url, {"a": 2}))
 
         pred = util.FilterPredicate(["a < 3", "b < 4", "c < 5"])
         self.assertTrue(pred(url, {"a": 2, "b": 3, "c": 4}))
         self.assertFalse(pred(url, {"a": 3, "b": 3, "c": 4}))
         self.assertFalse(pred(url, {"a": 2, "b": 4, "c": 4}))
         self.assertFalse(pred(url, {"a": 2, "b": 3, "c": 5}))
-        with self.assertRaises(exception.FilterError):
-            pred(url, {"a": 2})
+
+        self.assertFalse(pred(url, {"a": 2}))
 
     def test_build_predicate(self):
         pred = util.build_predicate([])
@@ -445,6 +444,7 @@ class TestOther(unittest.TestCase):
         self.assertEqual(expr({"a": 1, "b": 2, "c": 3}), 7)
         self.assertEqual(expr({"a": 9, "b": 9, "c": 9}), 90)
 
+        expr = util.compile_expression_raw("a + b * c")
         with self.assertRaises(NameError):
             expr()
         with self.assertRaises(NameError):