Add an InvocationError exception

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/reclass/errors.py b/reclass/errors.py
index 959f466..bb81e19 100644
--- a/reclass/errors.py
+++ b/reclass/errors.py
@@ -21,6 +21,12 @@
     rc = property(lambda self: self._rc)
 
 
+class InvocationError(ReclassException):
+
+    def __init__(self, msg, rc=posix.EX_USAGE):
+        super(InvocationError, self).__init__(msg, rc)
+
+
 class NotFoundError(ReclassException):
 
     def __init__(self, msg, rc=posix.EX_IOERR):