Add new exception MDInputTooLongException for overly long input strings

exceptions/generic/MDInputTooLongException.php

@@ -0,0 +1,20 @@
+<?PHP
+declare(strict_types = 1);
+
+/**
+ * Exception thrown in case of inputs longer than the allowed length of a
+ * field's contents in the DB.
+ */
+final class MDInputTooLongException extends MDgenericInvalidInputsException {
+    /**
+     * Error message.
+     *
+     * @return string
+     */
+    public function errorMessage() {
+        //error message
+        $errorMsg = 'Input string is too long';
+        return $errorMsg;
+
+    }
+}