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.

mercurial/error.py

@@ -177,6 +177,9 b' class PushRaced(RuntimeError):'
 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"""