Split index file into src/ folder components

src/errors.js

@@ -0,0 +1,13 @@
+class MissingTextError extends Error {
+  constructor(error = null) {
+    const message = `Missing query parameter 'text'`;
+    super(message);
+
+    this.error = error;
+    this.statusCode = 400;
+  }
+}
+
+module.exports = {
+  MissingTextError,
+};