[KSieve] Fix messagebox about connection to server lost

Summary:
If the TCP idle timeout kicks in, or we time out trying to connect,
and the user isn't manually doing something sieve related (no job),
this isn't worth bothering the user about.

This typically happens some time after starting kmail, for the initial
vacation check.

The 2019 Akademy university network was the ideal testcase for this
(due to blocked ports) but I also regularly get this elsewhere.

Test Plan:
start kmail, wait. There is no longer two annoying messageboxes
popping up (two because we have two sieve sessions at all times).

The lack of the word Sieve in the messageboxes made this even more irritating
(they were completely out of context) but that's for master (i18n change).

Reviewers: ervin, knauss, mlaurent, lbeltrame

Reviewed By: ervin, lbeltrame

Subscribers: lbeltrame, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D23820