mail: catch LookupError in headdecode()

We already catch this exception in _encode() (called by headencode()).
It gets raised when running test-notify.t with Python 3.

mercurial/mail.py

@@ -458,7 +458,7 b' def headdecode(s):'
             try:
                 uparts.append(part.decode(charset))
                 continue
-            except UnicodeDecodeError:
+            except (UnicodeDecodeError, LookupError):
                 pass
         # On Python 3, decode_header() may return either bytes or unicode
         # depending on whether the header has =?<charset>? or not