diff -r fb04372d7b38 -r a5192774e925 mercurial/error.py
--- a/mercurial/error.py	Fri Jun 05 13:31:18 2015 -0700
+++ b/mercurial/error.py	Wed May 27 23:48:54 2015 -0700
@@ -151,6 +151,21 @@
     """error raised when code tries to alter a part being generated"""
     pass
 
+class PushkeyFailed(Abort):
+    """error raised when a pushkey part failed to update a value"""
+
+    def __init__(self, partid, namespace=None, key=None, new=None, old=None,
+                 ret=None):
+        self.partid = partid
+        self.namespace = namespace
+        self.key = key
+        self.new = new
+        self.old = old
+        self.ret = ret
+        # no i18n expected to be processed into a better message
+        Abort.__init__(self, 'failed to update value for "%s/%s"'
+                       % (namespace, key))
+
 class CensoredNodeError(RevlogError):
     """error raised when content verification fails on a censored node