zlib+mbox: Fixed error handling when opening nonexistent/directory mailbox.
zlib+mbox: Fixed error handling when opening nonexistent/directory mailbox.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot