Custom error messages when register

[leoglobant]

Hello.

I was wondering if you could guide me on this.

I need to know how can I show a custom error message when the user is registering. As I need to check the user existence in other system before registering, and that system is going to provide me an error message, I need to show that error as if it was from vBulletin (wrong username, wrong email and that sort of thing)

Where should I look at?

Thanks a lot in advance.