Add exceptions special to this repo

exceptions/MDInvalidNodaLinkException.php

@@ -0,0 +1,18 @@
+<?PHP
+declare(strict_types = 1);
+
+/**
+ * Exception thrown in case a controlled vocabulary is to be linked using an invalid ID.
+ */
+class MDInvalidNodaLinkException extends MDgenericInvalidInputsException {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        return 'Inputs are not valid.';
+
+    }
+}

exceptions/MDInvalidNodaLinkLocIdIsSh.php

@@ -0,0 +1,21 @@
+<?PHP
+declare(strict_types = 1);
+
+/**
+ * Exception thrown in case a library of congress link refers to the subject
+ * headings (subject headings are a dedicated, full vocabulary according to
+ * md's definition).
+ */
+class MDInvalidNodaLinkLocIdIsSh extends MDInvalidNodaLinkException {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        return 'Inputs are not valid.';
+
+    }
+}
+