utils: 'find-files' always returns a proper list.

Reported at <http://bugs.gnu.org/15608>.

* guix/build/utils.scm (find-files): Change the 'error' procedure to
  return RESULT.  Before we would end up with an improper list.

guix/build/utils.scm

@@ -207,7 +207,7 @@ (define file-rx
                           (lambda (file stat errno result)
                             (format (current-error-port) "find-files: ~a: ~a~%"
                                     file (strerror errno))
-                            #f)
+                            result)
                           '()
                           dir)
         string<?))