A couple improvements

Show 400 if request is missing spec (instead of redirect)
Allow config.php to change factories for Minify and MinApp objects
These should've been separate commits, :(

lib/Minify.php

@@ -426,11 +426,16 @@ class Minify {
     }
 
     /**
-     * @param string $header
+     * Show an error page
      *
-     * @param string $url
+     * @param string $header E.g. 'HTTP/1.0 500 Internal Server Error'
+     * @param string $url    URL to direct the user to
+     *
+     * @return void
+     * @internal This is not part of the public API and is subject to change
+     * @access private
      */
-    protected function errorExit($header, $url)
+    public function errorExit($header, $url)
     {
         $url = htmlspecialchars($url, ENT_QUOTES);
         list(,$h1) = explode(' ', $header, 2);