Add exception for invalid currencies

exceptions/updates/MDInvalidCurrency.php

@@ -0,0 +1,29 @@
+<?PHP
+/**
+ * This file contains an exception class for usage of unaccepted currencies.
+ *
+ * @file
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Custom exception class to be raised when trying to use a currency that is not
+ * accepted by museum-digital.
+ */
+class MDInvalidCurrency extends MDgenericInvalidInputsException {
+
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        $errorMsg = 'The following currency is not available: ' . $this->getMessage();
+        return $errorMsg;
+
+    }
+
+}