如何正常显示数据库错误？ （例如SQLSTATE [23000]：...）

I have two unique indexes set on my users table columns (email, username) but when I try to register a new user with a taken email AND taken username, it displays an exception as expected. However, it just shows an exception on the email column, not the username:

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'martynbissett@yahoo.co.uk' for key 'email_2'

In my code I've tried doing something like this:

try {

    // attempt to save

} catch(\Exception $e) {

    switch($e->getCode()) {
        case 23000:

            $this->flash->error('Email(?) address already taken'); // or is it username?
            break;

        default:
            $this->flash->error($e->getMessage());
    }
}

Also, you can see I want to present a nicer error message than SQLSTATE[23000]: Integrity const... so I'm catching the error code. But I don't know in any given case which column is the problem ("Email address taken" or "Username taken"?)

Do I need to perform separate queries for each of these prior during validation? If I just had one unique index (e.g. only email) then I could just rely on the code in the exception, but if I have two....

Just do the validation before inserting to database. Queries to check if specified value exists are really cheap, so don't be afraid.

Database constraints will be additional protection.