fix inaccurate UI messages when some messages are excluded from the cached lists

Some messages were excluded from the copy/delete list after the UI message said
they were copied/deleted.
Also fix Internaldate2epoch(), which was wrong.

Signed-off-by: Janna Martl <janna.martl109@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Maildir.py

@@ -439,9 +439,6 @@ class MaildirFolder(BaseFolder):
         :return: Nothing, or an Exception if UID but no corresponding file
                  found.
         """
-        if not self.uidexists(uid):
-            return
-
         filename = self.messagelist[uid]['filename']
         filepath = os.path.join(self.getfullname(), filename)
         try: