mercurial/transaction.py

--- a/mercurial/transaction.py	Sun Mar 26 16:57:25 2017 -0700
+++ b/mercurial/transaction.py	Sun Mar 26 16:59:30 2017 -0700
@@ -226,7 +226,7 @@
         """
         if self._queue:
             msg = 'cannot use transaction.addbackup inside "group"'
-            raise RuntimeError(msg)
+            raise error.ProgrammingError(msg)
 
         if file in self.map or file in self._backupmap:
             return