Use error-display-handler instead of threads for multiple error reporting.

Closes PR 13240 (in reality, avoids the bug, but ryanc says that it's too
hard to fix).

collects/typed-racket/utils/tc-utils.rkt

@@ -93,9 +93,9 @@ don't depend on any other portion of the system
     [l
      (let ([stxs
             (for/list ([e l])
-              (sync (thread
+              (with-handlers ([exn:fail:syntax? 
-                     (lambda ()
+                               (λ (e) ((error-display-handler) (exn-message e) e))])
-                       (raise-typecheck-error (err-msg e) (err-stx e)))))
+                (raise-typecheck-error (err-msg e) (err-stx e)))
               (err-stx e))])
        (reset!)
        (unless (null? stxs)