Registration hook location help and custom error message

[Vitesse]

Hi guys/gals

Got a couple of questions for a ldification im doing on one of my sites, Im preventing users registering unless they have been issued with an individual 10 character code which is stored in a DB table, at present i've written a bit of simple code and added it to register.php that checks the code they entered on the registration form exists in the DB table and if so registration is processed or halted however I've tied it to use the "administrator has disabled registrations" message.

Unfortunately if certain other parts of the registration form are missing data it then removes the code from the DB before registering the member so I've obviously got my code in the wrong place.

I'm thinking of adding this as a plugin and wanted to check is this the correct hook location for it:
register_addmember_process

or if not does anyone know which one it is

Also can someone point me in the right direction for adding a custom error for registration?

Thanks

Tom