guix-download: Gracefully handle invalid URIs.

* guix-download.in (guix-download): Error out when `string->uri'
  returns #f.  Use `leave' when the scheme is unknown.
* tests/guix-download.sh: Add tests.

guix-download.in

@@ -159,16 +159,15 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
 
   (let* ((opts  (parse-options))
          (store (open-connection))
-         (uri   (string->uri (assq-ref opts 'argument)))
+         (uri   (or (string->uri (assq-ref opts 'argument))
+                    (leave (_ "guix-download: ~a: failed to parse URI~%")
+                           (assq-ref opts 'argument))))
          (fetch (case (uri-scheme uri)
                   ((http) http-fetch)
                   ((ftp)  ftp-fetch)
                   (else
-                   (begin
-                     (format (current-error-port)
-                             (_ "guix-download: ~a: unsupported URI scheme~%")
-                             (uri-scheme uri))
-                     (exit 1)))))
+                   (leave (_ "guix-download: ~a: unsupported URI scheme~%")
+                          (uri-scheme uri)))))
          (path  (call-with-temporary-output-file
                  (lambda (name port)
                    (fetch uri port)

tests/guix-download.sh

@@ -25,3 +25,9 @@ guix-download --version
 # Make sure it fails here.
 if guix-download http://www.example.com/does-not-exist
 then false; else true; fi
+
+if guix-download unknown://some/where;
+then false; else true; fi
+
+if guix-download not/a/uri;
+then false; else true; fi