fix error reporting in `reencode-output-port'

Closes PR 12077

collects/mzlib/port.rkt

@@ -1469,7 +1469,7 @@
                 [convert-newlines-to #f]
                 [decode-error (lambda (msg port)
                                 (error 'reencode-input-port
-                                       (format "~a: ~e" msg)
+                                       (format "~a: ~~e" msg)
                                        port))])
     (let ([c (bytes-open-converter "UTF-8" encoding)]
           [ready-bytes (make-bytes 1024)]

collects/tests/racket/portlib.rktl

@@ -726,7 +726,12 @@
  (port->bytes (reencode-input-port (open-input-bytes #"\xFF\xFF") "utf-8"))
  (lambda (exn)
    (regexp-match? #rx"^reencode-input-port:" (exn-message exn))))
-    
+(err/rt-test
+ (let ([o (reencode-output-port (open-output-bytes) "utf-8")])
+   (display #"\xFF\xFF" o) (flush-output o))
+ (lambda (exn)
+   (regexp-match? #rx"^reencode-output-port:" (exn-message exn))))
+
 ;; --------------------------------------------------
 
 (let-values ([(in out) (make-pipe)])