FailureNote JSON value is not useful

[ehuss]

Currently the FailureNote diagnostic level serializes to the empty string. This results in JSON messages containing level: "", which I'm not sure how to interpret.

This was added in #48684, though reading that PR I can't really figure out why it was added and how it relates to compiletest changes.

An easy way to see this is to trigger a diagnostic that prints "For more information…", for example:

// E0005
let Some(_y) = Some(1);

will emit a JSON value like this:

{
  "message": "For more information about this error, try `rustc --explain E0005`.",
  "code": null,
  "level": "",
  "spans": [],
  "children": [],
  "rendered": "For more information about this error, try `rustc --explain E0005`.\n"
}

The empty string is not very descriptive of how to handle the message. I would suggest having it serialize to something more deliberate, such as "failure-note". I can't really tell what the intent of FailureNote is, so it's hard to say what it should be.

cc @GuillaumeGomez who might know more.

[GuillaumeGomez]

Serialising as "failure-note" sounds good to me. Weird we didn't do it beforehand...

[afnanenayet]

I'd like to pick this up