error: introduce new InterventionRequired exception

Future changes will use this type instead of util.Abort for signalling
the user that intervention is required, as in some rebase and histedit
aborts.

mercurial/error.py

@@ -33,6 +33,9 b' class ManifestLookupError(LookupError):'
 class CommandError(Exception):
     """Exception raised on errors in parsing the command line."""
 
+class InterventionRequired(Exception):
+    """Exception raised when a command requires human intervention."""
+
 class Abort(Exception):
     """Raised if a command needs to print an error and exit."""
     def __init__(self, *args, **kw):