Optionally email the admin if a send task failed and we suspend the source cleanup

Should send the error summary to specified recipient(s) and fix the issue #499 as far as I (original poster) am concerned :)

Tested on a local system, delivers to one or two (comma-separated) local mailboxes using the standard /usr/lib/sendmail or /usr/sbin/sendmail alias (exim, postfix also provide that interface). Internet delivery is up to MTA setup and outside znapzend's interest.

:; znapzend --noaction --runonce=nvpool/SHARED/var/test --inherited --debug --cleanOffline --mailErrorSummaryTo=root,jim

...
[2020-08-20 00:29:08.80293] [26384] [warn] ERROR: 1 send task(s) below failed for nvpool/SHARED/var/test, but "cleanOffline" mode is on, so proceeding to clean up source dataset carefully:
[2020-08-20 00:29:08.80315] [26384] [warn]  +-->   destination 'backup-adata/snapshots/nvpool/SHARED/var/test' does not exist or is offline. ignoring it for this round...
[2020-08-20 00:29:08.80643] [26384] [warn] Sending a copy of the report above to root,jim
[2020-08-20 00:29:08.83905] [26384] [debug] checking to clean up snapshots recursively from source nvpool/SHARED/var/test
...

If the mail program does not exist in standard path (currently hardcoded, can be made an argument later), it is noted but not fatal:

[2020-08-20 00:27:28.54081] [26359] [warn] ERROR: 1 send task(s) below failed for nvpool/SHARED/var/test, but "cleanOffline" mode is on, so proceeding to clean up source dataset carefully:
[2020-08-20 00:27:28.54089] [26359] [warn]  +-->   destination 'backup-adata/snapshots/nvpool/SHARED/var/test' does not exist or is offline. ignoring it for this round...
Can't exec "/usr/sbin/sendmailsss": No such file or directory at /export/home/jim/shared/znapzend/bin/../lib/ZnapZend.pm line 786.
Can't open /usr/sbin/sendmailsss to send a copy of the report above to root,jim!

Message in the box:

From jim@jimoi.local Thu Aug 20 00:29:08 2020
Return-Path: <jim@jimoi.local>
Received: from jimoi.local (jimoi [127.0.0.1])
        by jimoi.local (8.15.2+Sun/8.15.2) with ESMTP id 07JMT880026388;
        Thu, 20 Aug 2020 00:29:08 +0200 (CEST)
Received: (from jim@localhost)
        by jimoi.local (8.15.2+Sun/8.15.2/Submit) id 07JMT8wN026387;
        Thu, 20 Aug 2020 00:29:08 +0200 (CEST)
Date: Thu, 20 Aug 2020 00:29:08 +0200 (CEST)
From: jim <jim@jimoi.local>
Message-Id: <202008192229.07JMT8wN026387@jimoi.local>
To: root@jimoi.local, jim@jimoi.local
Subject: znapzend replication error summary
Content-Length: 290

-------
ERROR: 1 send task(s) below failed for nvpool/SHARED/var/test, but "cleanOffline" mode is on, so proceeding to clean up source dataset carefully:
 +-->   destination 'backup-adata/snapshots/nvpool/SHARED/var/test' does not exist or is offline. ignoring it for this round...
-------

oetiker / znapzend

Optionally email the admin if a send task failed and we suspend the source cleanup #520