Improved error and success emails formating, and included missing general borg logs in them

[eauchat]

Problem

1. There are missing logs in the error email sent to admins, for example, if a repo's quota has been reached, this is logged in the general /var/log/borg/borg.log, and not in the operation log, so the admin won't see about it in the email and therefore have a harder time figuring out why backup failed.
2. Emails could be more clear, there are logs from two different files but it looks like one single log, so one does not understand the chronology of the logs.

Solution

1. Include last logs from /var/borg/<app>/borg.log in error emails.
2. Improved the formatting of sent emails.

PR Status

• Code finished and ready to be reviewed/tested
• The fix/enhancement were manually tested (if applicable)

Automatic tests

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