Add class MDPageNotAvailable for functionalities disabled by config

exceptions/page/MDPageNotAvailable.php

@@ -0,0 +1,19 @@
+<?PHP
+declare(strict_types = 1);
+
+/**
+ * Exception thrown in case the main entity of a page (e.g. an object, an institution etc.)
+ * are not set public.
+ */
+final class MDPageNotAvailable extends MDExpectedException {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        return 'There is no entity to be shown here (id: <b>' . $this->getMessage() . '</b>).';
+
+    }
+}