fix error reporting in `reencode-input-port'

Closes PR 12076

collects/mzlib/port.rkt

@@ -1332,7 +1332,7 @@
                 [newline-convert? #f]
                 [decode-error (lambda (msg port)
                                 (error 'reencode-input-port
-                                       (format "~a: ~e" msg)
+                                       (format "~a: ~~e" msg)
                                        port))])
     (let ([c (let ([c (bytes-open-converter encoding "UTF-8")])
                (if newline-convert? (mcons c #f) c))]

collects/tests/racket/portlib.rktl

@@ -722,6 +722,11 @@
     (test #\c read-char ei)
     (test eof read-char ei)))
 
+(err/rt-test
+ (port->bytes (reencode-input-port (open-input-bytes #"\xFF\xFF") "utf-8"))
+ (lambda (exn)
+   (regexp-match? #rx"^reencode-input-port:" (exn-message exn))))
+    
 ;; --------------------------------------------------
 
 (let-values ([(in out) (make-pipe)])