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 rust-lang#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
```

Author: nnethercote

compiler/rustc_session/src/config.rs

@@ -2051,23 +2051,14 @@ fn check_error_format_stability(
     early_dcx: &mut EarlyDiagCtxt,
     unstable_opts: &UnstableOptions,
     error_format: ErrorOutputType,
-    json_rendered: HumanReadableErrorType,
 ) {
     if !unstable_opts.unstable_options {
-        if let ErrorOutputType::Json { pretty: true, json_rendered } = error_format {
-            early_dcx.abort_if_error_and_set_error_format(ErrorOutputType::Json {
-                pretty: false,
-                json_rendered,
-            });
+        if let ErrorOutputType::Json { pretty: true, .. } = error_format {
             early_dcx.early_fatal("`--error-format=pretty-json` is unstable");
         }
         if let ErrorOutputType::HumanReadable(HumanReadableErrorType::AnnotateSnippet(_)) =
             error_format
         {
-            early_dcx.abort_if_error_and_set_error_format(ErrorOutputType::Json {
-                pretty: false,
-                json_rendered,
-            });
             early_dcx.early_fatal("`--error-format=human-annotate-rs` is unstable");
         }
     }
@@ -2665,7 +2656,7 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
     let mut unstable_opts = UnstableOptions::build(early_dcx, matches);
     let (lint_opts, describe_lints, lint_cap) = get_cmd_lint_options(early_dcx, matches);
 
-    check_error_format_stability(early_dcx, &unstable_opts, error_format, json_rendered);
+    check_error_format_stability(early_dcx, &unstable_opts, error_format);
 
     if !unstable_opts.unstable_options && json_unused_externs.is_enabled() {
         early_dcx.early_fatal(