fix: correct Mailgun API URL and enhance error handling in MailgunAccount class

- Fixed the incorrect API URL in MailgunAccount class.
- Improved error handling for email sending failures by providing detailed error messages based on response content.
- Updated test file to export the tap.start() function for better module integration.

test/test.ts

@@ -66,4 +66,4 @@ tap.test('should retrieve a mail using a retrieval url', async () => {
   }
 });
 
-tap.start();
+export default tap.start();