Allow msg to be specified for Maybe

This makes it possible to supply a good error message to the user.
This got reported in #369

voluptuous/tests/tests.py

@@ -624,6 +624,12 @@ def test_maybe():
     assert_raises(Invalid, s, {'foo': 'bar'})
 
 
+def test_maybe_accepts_msg():
+    s = Schema(Maybe(int, msg='int or None expected'))
+    with raises(MultipleInvalid, 'int or None expected'):
+        assert s([])
+
+
 def test_empty_list_as_exact():
     s = Schema([])
     assert_raises(Invalid, s, [1])

voluptuous/validators.py

@@ -487,7 +487,7 @@ def PathExists(v):
         raise PathInvalid("Not a Path")
 
 
-def Maybe(validator):
+def Maybe(validator, msg=None):
     """Validate that the object matches given validator or is None.
 
     :raises Invalid: if the value does not match the given validator and is not
@@ -500,7 +500,7 @@ def Maybe(validator):
     ...  s("string")
 
     """
-    return Any(None, validator)
+    return Any(None, validator, msg=msg)
 
 
 class Range(object):