Ragged, semi-tested; been a long time I really stepped through this so consider this cargo-culting and refer to the docs; Catalyst::Action::RenderView. This idiom is common and useful if you don’t go to the trouble of getting the Plugin::CustomErrorMessage stuff set up.

# In MyApp::Controller::Root…

sub render :ActionClass("RenderView") {}

sub end :Private {
    my ( $self, $c ) = @_;
    return if $c->response->body; # Stop, good or bad, it's been rende
+red.
    $c->forward("render"); # Send to RenderView.

    # If there was an error in the render above, process it and just o
+utput the error.
    if ( my @err = @{ $c->error } )
    {
        $c->log->error("Error: " . join(", ", @err));
        $c->response->status(503);
        $c->response->body("There was an unrecoverable error: " .  joi
+n(", ", @err));
        # ^^^ Could contain user input like XSS attacks, so naïve/dang
+erous as shown.
        # Echoing errors is a security problem in itself.
        $c->clear_errors;
    }
}
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Update 20211102: added a comment about danger of echoing errors and insecurity of sample code.