[PATCH 1 of 3 V2] error: add a new exception named WdirUnsupported

[Pulkit Goyal]

# HG changeset patch
# User Pulkit Goyal <7895pulkit at gmail.com>
# Date 1495201237 -19800
#      Fri May 19 19:10:37 2017 +0530
# Node ID 2adbd2d46cd7588d75201108f6e02b92d1480022
# Parent  4a23cdb32968ab4739cad0c32811b24829c38efa
error: add a new exception named WdirUnsupported

This series intents to support wdir() predicate with different operators
like ~, ^.

Currently the storage class don't support wdir(). This exception is introduced
so that it can be raised from places where wdir() is passed and is not
supported and we can catch this where we want to support the predicate.
Also throwing an error at low level and catching at some higher level is better
than using if-else especially for perf.

diff --git a/mercurial/error.py b/mercurial/error.py
--- a/mercurial/error.py
+++ b/mercurial/error.py
@@ -177,6 +177,9 @@
 class ProgrammingError(Hint, RuntimeError):
     """Raised if a mercurial (core or extension) developer made a mistake"""
 
+class WdirUnsupported(Exception):
+    """An exception which is raised when 'wdir()' is not supported"""
+
 # bundle2 related errors
 class BundleValueError(ValueError):
     """error raised when bundle2 cannot be processed"""