Error message missing if output is not a string

The following script should show an exception that the output is not a string, or maybe convert the result to a string:

```racket
#lang racket/base
(require quickscript)

(define-script sloc
  #:label "foo"
  #:output-to message-box
  (λ (selection)
    0))
```

Currently, it just doesn't show a message box at all, and fails to indicate it fails.
