Return MAPISTORE_ERR_NOT_FOUND when a mail is indexed but not in IMAP server

And remove that entry from the indexing table.

This avoids to crash getting properties from a no longer available message
in the IMAP server, for instance, the `PidTagPredecessorChangeList` attribute.
Merge pull request #58 from Zentyal/ejhernandez/open-msg-not-found-imap

Return MAPISTORE_ERR_NOT_FOUND when a mail is indexed but not in IMAP server