Super gross mode no longer sync throws flattened errors

[benlesh]

Given this code:

import { throwError, of, config } from "rxjs";
import { concatMap } from "rxjs/operators";

console.clear();

config.useDeprecatedSynchronousErrorHandling = true;

try {
  of(1)
    .pipe(concatMap(() => throwError(new Error("test"))))
    .subscribe(console.log);
} catch (err) {
  console.log("Caught!", err);
}

It should log "Caught!" and the error, but it does not.

Related: #5979

cc @leggechr