--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -136,6 +136,9 @@ def unique(g):
 class Abort(Exception):
     """Raised if a command needs to print an error and exit."""
 
+class UnexpectedOutput(Abort):
+    """Raised to print an error with part of output and exit."""
+
 def always(fn): return True
 def never(fn): return False