Add exception to be thrown when an invalid PGP key is submitted

exceptions/updates/MDPgpKeyIsInvalid.php

@@ -0,0 +1,26 @@
+<?PHP
+/**
+ * This file contains an exception class to be raised if a submitted PGP key
+ * is invalid.
+ *
+ * @file
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Custom exception class to be raised if a submitted PGP key is not valid.
+ */
+final class MDPgpKeyIsInvalid extends MDgenericInvalidInputsException {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        return 'PGP key is invalid. ' . $this->getMessage();
+
+    }
+}