MDL-50788 classloader: Prevent cryptic errors on unreadable classes dir

Do not attempt to read an unreadable directory because previously the generated
errors were hard to fathom.

Thanks to James McQuillan for the basis of the patch.

lib/classes/component.php

@@ -651,6 +651,13 @@ protected static function load_classes($component, $fulldir, $namespace = '') {
             return;
         }
 
+        if (!is_readable($fulldir)) {
+            // TODO: MDL-51711 We should generate some diagnostic debugging information in this case
+            // because its pretty likely to lead to a missing class error further down the line.
+            // But our early setup code can't handle errors this early at the moment.
+            return;
+        }
+
         $items = new \DirectoryIterator($fulldir);
         foreach ($items as $item) {
             if ($item->isDot()) {