[voltLib] Add VoltLibError

Lib/fontTools/voltLib/error.py

@@ -0,0 +1,16 @@
+from __future__ import print_function, division, absolute_import
+from __future__ import unicode_literals
+
+
+class VoltLibError(Exception):
+    def __init__(self, message, location):
+        Exception.__init__(self, message)
+        self.location = location
+
+    def __str__(self):
+        message = Exception.__str__(self)
+        if self.location:
+            path, line, column = self.location
+            return "%s:%d:%d: %s" % (path, line, column, message)
+        else:
+            return message