Output of restic version

restic 0.16.4 compiled with go1.22.2 on linux/amd64

What should restic do differently? Which functionality do you think we should add?

Currently, fatal errors aren't JSON formatted if --json is provided. It would be nice if it did surface that in a more programmatically-detectable way.

What are you trying to do? What problem would this solve?

Fatal errors can be detected by scanning for lines that start with Fatal: but still... it'd make me feel safer if I could instead scan for a JSON blob with "message_type": "fatal_error" though.

This would be especially handy if Restic ever wants to add more metadata to the error (like requested in #4947)

Open Questions

• What should the message_type be? I could see an argument for fatal_error but I could also see an argument for just error (also used for per-file errors in backup/restore commands) with either a null path or a flag like "is_fatal: true`.
• Should we keep printing the Fatal: freeform string to the console, to ease the transition for folks like me that are already looking for and parsing such lines?

I'm happy to work on a PR for this, if I could get some pre-guidance on the open questions. Thanks!