better error msg when token type is whitespace (fixes #9)

brag/private/internal-support.rkt

@@ -28,7 +28,10 @@
    (lambda (tok-type tok-value offset line column span)
      (raise (exn:fail:parsing
              (string-append
-              (format "Encountered unexpected token ~e (~e) while parsing" tok-type tok-value)
+              (format "Encountered unexpected token of type ~e (value ~e) while parsing"
+                      (if (memq tok-type (map string->symbol '("\n" "\t" "\r")))
+                          (format "~a" tok-type)
+                          tok-type) tok-value)
               (if (or (current-source) line column offset)
                   (format " ~e [line=~a, column=~a, offset=~a]" (current-source) line column offset)
                   ""))

brag/test/test-0n1.rkt

@@ -46,5 +46,5 @@
 (check-exn exn:fail:parsing? 
            (lambda () (parse '("zero" "one" "zero"))))
 (check-exn (regexp (regexp-quote
-                    "Encountered unexpected token \"zero\" (\"zero\") while parsing"))
+                    "Encountered unexpected token of type \"zero\" (value \"zero\") while parsing"))
            (lambda () (parse '("zero" "one" "zero"))))