Maildir: OfflineImapError is missing the severity argument

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/350
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Maildir.py

@@ -462,7 +462,8 @@ class MaildirFolder(BaseFolder):
         """
 
         if not uid in self.messagelist:
-            raise OfflineImapError("Cannot change unknown Maildir UID %s"% uid)
+            raise OfflineImapError("Cannot change unknown Maildir UID %s"% uid,
+                                   OfflineImapError.ERROR.MESSAGE)
         if uid == new_uid: return
 
         oldfilename = self.messagelist[uid]['filename']