diff -r a5a06c9c7407 -r a450e0a2ba0a mercurial/error.py
--- a/mercurial/error.py	Wed Jan 21 16:35:09 2015 -0500
+++ b/mercurial/error.py	Fri Feb 06 00:55:29 2015 +0000
@@ -141,3 +141,11 @@
     def __init__(self, filename, node):
         from node import short
         RevlogError.__init__(self, '%s:%s' % (filename, short(node)))
+
+class CensoredBaseError(RevlogError):
+    """error raised when a delta is rejected because its base is censored
+
+    A delta based on a censored revision must be formed as single patch
+    operation which replaces the entire base with new content. This ensures
+    the delta may be applied by clones which have not censored the base.
+    """