Add exception for invalid mail addresses

exceptions/generic/MDInvalidEmail.php

@@ -0,0 +1,22 @@
+<?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.
+ */
+class MDInvalidEmail extends MDgenericInvalidInputsException {
+
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        $errorMsg = 'Input URL is invalid';
+        return $errorMsg;
+
+    }
+
+}