Add AccountsAPIError class and productvoucher route

Introduces a custom AccountsAPIError class for consistent error handling and serialization. Adds a new /productvoucher/giftcode route that returns a 404 error response for unimplemented endpoints.

errors/ServiceError.js

@@ -0,0 +1,32 @@
+class AccountsAPIError extends Error {
+    constructor(code, path, error, errorMessage) {
+        super(errorMessage || error || "Accounts API Error")
+        this.code = code
+        this.path = path
+        this.errorType = error
+        this.errorMessage = errorMessage
+        this.isOperational = true
+        
+        Error.captureStackTrace(this, this.constructor)
+    }
+
+    serialize() {
+        const response = {}
+
+        if (this.path && this.path.trim() !== "") {
+            response.path = this.path.replace(/:(\w+)/g, "<$1>")
+        }
+
+        if (this.errorType && this.errorType.trim() !== "") {
+            response.error = this.errorType
+        }
+
+        if (this.errorMessage && this.errorMessage.trim() !== "") {
+            response.errorMessage = this.errorMessage
+        }
+
+        return response
+    }
+}
+
+module.exports = AccountsAPIError