Change the error message "An illegal choice has been detected. Please contact the site administrator." When a user submits a content that has a taxonomy selection field and NOT make a selection, then the referenced error appears.

Now, I know exactly when, how and where the message appears, is there any way to change it so that it's actually useful for the user (i.e. "You did not select a category."), and not merely a cryptic error message that could potentially freak the user out?
This is to add onto Letharion's answer. Using the same principle, we can also check for error messages to replace in the validator.

function MY_MODULE_form_validate($form, &amp;$form_state){
    $errors = drupal_get_messages(); // (dpm()'s has to go AFTER this line--they get cleared)
    foreach ($errors as $type =&gt; $id) {
        foreach ($id as $message){
            // Loop through individual messages, looking for ones to remove or replace
            if (test_for_invalid_error($message)===FALSE){
                drupal_set_message($message,$type);
            } elseif (test_for_invalid_error($message)!==TRUE){
                drupal_set_message(test_for_invalid_error($message), $type);
            }
        }
    }
}

function test_for_invalid_error($message){
    if (strpos($message, 'An illegal choice has been detected. Please contact the site administrator.') !== false) {
        return t('Please select a category!');
    }
    return FALSE;
}


This will not only add your own custom error, but replace the "An illegal choice" error. Isn't it neat? Yay!