Fix #352: Limit the length of backup logs for mails

Fix #352: Limit the length of backup logs for mails

When creating a backup the log part can make the mail too big to be
transferred. To ensure delivery, two measures are taken:
1. Always omit the status lines
2. Omit the whole log part if a mail becomes (too) big

Additionally, add a check for missing log files.

Co-developed-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Dominic Jäger <d.jaeger@proxmox.com>