Change printing of "`--error-format` is unstable" errors.

Currently for these two errors we go to the effort of switching to a
standard JSON emitter, for no obvious reason, and unlike any other
errors. This behaviour was added for `pretty-json` in #45737, and then
`human-annotate-rs` copied it some time later when it was added.

This commit changes things to just using the requested emitter, which is
simpler and consistent with other errors.

Old output:
```
$ rustc --error-format pretty-json
{"$message_type":"diagnostic","message":"`--error-format=pretty-json` is unstable","code":null,"level":"error","spans":[],"children":[],"rendered":"error: `--error-format=pretty-json` is unstable\n\n"}

$ rustc --error-format human-annotate-rs
{"$message_type":"diagnostic","message":"`--error-format=human-annotate-rs` is unstable","code":null,"level":"error","spans":[],"children":[],"rendered":"error: `--error-format=human-annotate-rs` is unstable\n\n"}
```

New output:
```
$ rustc --error-format pretty-json
{
  "$message_type": "diagnostic",
  "message": "`--error-format=pretty-json` is unstable",
  "code": null,
  "level": "error",
  "spans": [],
  "children": [],
  "rendered": "error: `--error-format=pretty-json` is unstable\n\n"
}

$ rustc --error-format human-annotate-rs
error: `--error-format=human-annotate-rs` is unstable
```