fix `delay/thread' error when promise-running thread terminates

collects/racket/promise.rkt

@@ -98,9 +98,13 @@
   (lambda (p)
     (reify-result (let ([v (pref p)])
                     (if (running-thread? v)
-                      (begin (thread-wait (running-thread-thread v))
-                             (pref p))
-                      v))))
+                        (let ([t (running-thread-thread v)])
+                          (thread-wait t)
+                          (let ([v (pref p)])
+                            (if (running-thread? v)
+                                (error 'force "promise's thread terminated without result or exception\n  promise: ~e" p)
+                                v)))
+                        v))))
   #:property prop:evt
   (lambda (p)
     (let ([v (pref p)])

collects/scribblings/reference/promise.scrbl

@@ -111,7 +111,10 @@ group} specified by @racket[thread-group-expr], which defaults to
 appear at most once.
 
 Exceptions raised by the @racket[body]s are caught as usual and raised
-only when the promise is @racket[force]d.}
+only when the promise is @racket[force]d. Unlike @racket[delay/sync],
+if the thread running @racket[body] terminates without producing a result
+or exception, @racket[force] of the promise raises an exception (instead
+of blocking).}
 
 @defform/subs[(delay/idle body/option ...+)
               ([body/option body