Add default message attribute

The `message` attribute on ValueError has been deprecated since 2.6 and removed in Python 3. Explicitly add it ourselves for our subclasses. Fixes #2

fwffr.py

@@ -8,7 +8,6 @@
 
 PY3 = sys.version_info[0] == 3
 
-__all__ = [
     'FixedLengthError',
     'FixedLengthUnknownRecordTypeError',
     'FixedLengthSeparatorError',
@@ -26,6 +25,9 @@ def iteritems(d, **kw):
 
 class FixedLengthError(ValueError):
     """ Base class for parsing errors """
+
+    def __init__(self, message=None):
+        self.message = message
 
     def __str__(self):
         return self.message