Return backup notification errors

[ShiroKSH]

Summary

• Return the Panel notification error when a generated backup cannot be marked successful.
• Keep the existing cleanup behavior that removes the archive after that notification fails.
• Add regression coverage for the successful-generation, failed-notification path.

Tests

• GOOS=linux GOARCH=amd64 GOPROXY=direct go test -c ./server

[ShiroKSH]

@pterodactyl We're ready for verification when you have a chance.