Allow format checkers to raise custom ValidationErrors

jsonschema.py

@@ -293,12 +293,15 @@ def validate_pattern(self, patrn, instance, schema):
             yield ValidationError("%r does not match %r" % (instance, patrn))
 
     def validate_format(self, format, instance, schema):
-        if (
-            self.format_checker is not None and
-            self.is_type(instance, "string") and
-            not self.format_checker.conforms(instance, format)
-        ):
-            yield ValidationError("%r is not a %r" % (instance, format))
+        try:
+            if (
+                self.format_checker is not None and
+                self.is_type(instance, "string") and
+                not self.format_checker.conforms(instance, format)
+            ):
+                yield ValidationError("%r is not a %r" % (instance, format))
+        except ValidationError as e:
+            yield e
 
     def validate_minLength(self, mL, instance, schema):
         if self.is_type(instance, "string") and len(instance) < mL:

tests.py

@@ -161,6 +161,16 @@ def test_it_validates_formats_if_a_checker_is_provided(self):
         with self.assertRaises(ValidationError):
             validator.validate("bar")
 
+    def test_it_raises_custom_errors_from_format_checkers(self):
+        message = "This is the custom error"
+        checker = mock.Mock(spec=FormatChecker)
+        validator = self.validator_class(
+            {"format": "foo"}, format_checker=checker
+        )
+        checker.conforms.side_effect = ValidationError(message)
+        with self.assertRaisesRegexp(ValidationError, message):
+            validator.validate("bar")
+
 
 @load_json_cases(
     "draft3/*.json", ignore_glob=os.path.join("draft3", "refRemote.json")