Pages templates, have_posts(), and 404.php

My site makes extensive use of page templates to render different classes of pages. Each page has some code that looks like the following for dealing with what happens when a page cannot be found:

<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>

... stuff for displaying the page ...

<?php endwhile; ?>

<?php else : ?>

... stuff for telling the user the page can't be found ...
Sorry, but you are looking for something that isn't here. You can search again by using 

<?php endif; ?>

Recently I wanted to change this message, but I don’t want to have to hunt it down in 20 different page templates and keep them all consistent. I’m assuming that’s what a “404.php” template would be for…

Now, to my question…. Are the conditions that cause 404.php and !have_posts() to be true the same? Can I safely eliminate the !have_posts() code from my page templates provided I supply a 404.php? It seems like a great way to clean up the page templates and make them simpler.

Also, is there an action hook or a filter where I can intercept page not found errors? I’m considering writing a plugin to deal with them.

Thanks,
Scott