keep our backups after a group is deleted on Telegram

[felipesanches]

user scenario:

• A user keeps backups of a group
• The owner of the group decides to delete it on Telegram
• the user re-runs the backup tool
• the backup tool does not find the group (because it does not exist anymore) and as a result it removes the backups from the generated output.

reasonable expected behaviour:

• simply ignore the fact that the group is not available on Telegram
• continue including the backups of the group in the generated output

ideal behaviour:

• optionally display on the generated HTML a notice that the group was deleted, but still display the contents of the backup
• It would be neat to have 2 sections in the HTML: one for the backup logs of the currently active groups and another one for the historical logs of groups that were deleted on Telegram.