introduced new exception classes for structured error reporting

src/errors/not_found_error.js

@@ -0,0 +1,7 @@
+class NotFoundError {
+    constructor(message) {
+        this.message = message;
+    }
+}
+
+module.exports = NotFoundError;

src/errors/validation_error.js

@@ -0,0 +1,7 @@
+class ValidationError {
+    constructor(message) {
+        this.message = message;
+    }
+}
+
+module.exports = ValidationError;