Add exceptions from musdb

exceptions/updates/MDDuplicateLinkException.php

@@ -0,0 +1,23 @@
+<?PHP
+declare(strict_types = 1);
+
+/**
+ * Exception thrown in case an erroneously duplicate link is to be entered in a
+ * linking table.
+ */
+class MDDuplicateLinkException extends Exception {
+
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+
+        //error message
+        $errorMsg = 'Duplicate link exception: ' . $this->getMessage() . '.';
+        return $errorMsg;
+
+    }
+
+}

exceptions/updates/MDwriteAccessDeniedException.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 MDwriteAccessDeniedException extends Exception {
+
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        $errorMsg = 'You have no permission to write this.';
+        return $errorMsg;
+
+    }
+
+}