How to get 404 exception message inside template file?

[abdus]

While coding some templates I need to throw some Wire404Exceptions, which, expectedly, sets correct headers and renders 404 page. Since all these exceptions are extending Exception class, I can call its constructor with a message like this

<?php
$demoName = sanitizer()->pageName(input()->urlSegment1);
$demo = pages('template=demo, name=$sanitizedPagenameFrom');

if(!$demo->id) {
	throw new Wire404Exception("Demo named $demoName cannot be found, but check out its source page instead");
}

// inside template 404.php

// somehow get the error $message
echo "<h1>Page does not exist</h1>"
echo "<p>$message</p>"

What I want to do is to somehow catch this exception, or get inside 404 template the message that I set earlier in some module or another function, so that I can notify the user with a better message than "Error 404: Page not found".

There's some parts inside core that handles this sort of things but they're not utilized. 

<?php
// inside /wire/core/ProcessPageView.php

public function ___execute($internal = true) {
    // ...
    try {
	    // try to render the page
    } catch(Wire404Exception $e) {
        return $this->pageNotFound($page, $this->requestURL, false, '404 thrown during render');
    }
    // ...
}

protected function ___pageNotFound($page, $url, $triggerReady = false, $reason = '') {
	// reason is not used, or passed to 404 template
}

So, is there a way to manually catch the exception thrown? For instance, with Silex all exceptions can be intercepted using

<?php
$app = new Silex\Application();
$app->error(function (\Exception $e, $code) {
    exit('asd');
});

 

[abdus]

Huh, turns out there's this method inside ProcessPageView class

<?php
// /wire/core/ProcessPageView.php
	/**
	 * Hook called when the pageview failed to finish due to an exception.
	 *
	 * Sends a copy of the exception that occurred.
	 *
 	 */
	public function ___failed(\Exception $e) {
		$this->wire()->setStatus(ProcessWire::statusFailed);
	}

which I should be able to hook and access the exception and message with

<?php
wire()->addHookBefore('ProcessPageView::failed', function(HookEvent $e) {
	$ex = $e->arguments(0);
	$message = $ex->getMessage();
})

But, it's not firing when hooked inside /site/ready.php. In fact it does not appear to be used anywhere inside the core at all.