error: add `hint` attribute to `SidedataHashError`

This prevents an exception within an exception because `hint` does not exist.

Differential Revision: https://phab.mercurial-scm.org/D10024

mercurial/error.py

@@ -55,6 +55,7 b' class RevlogError(StorageError):'
 
 class SidedataHashError(RevlogError):
     def __init__(self, key, expected, got):
+        self.hint = None
         self.sidedatakey = key
         self.expecteddigest = expected
         self.actualdigest = got