Added new exception type for configuration file errors

Change-Id: I3fadf3034cac0b1233ad12e2bc143a880bb93c01
diff --git a/errors.py b/errors.py
index 7093dd6..130ead6 100644
--- a/errors.py
+++ b/errors.py
@@ -32,3 +32,8 @@
     Invalid input
     """
 
+class ConfigError(VlandError):
+    """
+    Invalid configuration
+    """
+