Add specific exception in case of GPG encryption failing

exceptions/MDMailerEncryptionFailedException.php

@@ -0,0 +1,18 @@
+<?PHP
+declare(strict_types = 1);
+
+/**
+ * Exception to be thrown
+ */
+final class MDMailerEncryptionFailedException {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        return 'GPG encryption failed';
+
+    }
+}