Handle Task::NotFoundError with a 404 page instead of raising an error

[kaibadash]

• Add rescue_from Task::NotFoundError in ApplicationController to render a friendly 404 page when users access a task that no longer exists
• Create a dedicated not found view (not_found.html.erb) styled with Bulma to match the existing UI, with a link back to the tasks list
• Remove the rescue_responses mapping from Engine since the error is now handled directly by the controller

Motivation

Maintenance tasks are short-lived by nature — they are created for one-time data migrations or cleanup operations and are expected to be deleted once they are no longer needed. However, users may still have bookmarked URLs or links in Slack/docs pointing to deleted tasks.

Previously, accessing a deleted task with no associated runs raised MaintenanceTasks::Task::NotFoundError, which:

1. Displayed an unfriendly error page to users
2. Was reported to error monitoring systems (e.g., Sentry) as an unhandled exception, creating noise

By using rescue_from in the controller, the exception is caught before it propagates to error monitoring middleware, and users see a clean 404 page with navigation back to the task list.

[etiennebarrie]

2. Was reported to error monitoring systems (e.g., Sentry) as an unhandled exception, creating noise

This shouldn't be the case, since we declare NotFoundError as a rescuable exception that results in a 404:

maintenance_tasks/lib/maintenance_tasks/engine.rb

Lines 36 to 38 in 4b8ef60

	config.action_dispatch.rescue_responses.merge!(
	"MaintenanceTasks::Task::NotFoundError" => :not_found,
	)

$ bin/production &
$ curl -s -D - http://127.0.0.1:3000/maintenance_tasks/tasks/DoesNotExist | grep 404
HTTP/1.1 404 Not Found
  <title>The page you were looking for doesn't exist (404)</title>
  <!-- This file lives in public/404.html -->

Maybe Sentry's code does still mark those as unhandled exceptions, but then it's a bug/feature in their code.

[etiennebarrie]

I think this could be nice.

At first I thought that shouldn't be necessary (easy enough to tweak the URL), and the Sentry issue is a problem with that gem or the configuration (although it shouldn't be necessary to add this exception class to list of excluded exceptions, doing so would solve that issue).

But the change is small, the page looks nicer (and also the main app's 404 page might be super custom) and moving away from action_dispatch.rescue_responses means better support for exception monitoring tools which don't respect action_dispatch.report_exception.

[etiennebarrie]

I wish we could explicitly test the 404 but we can't use page.status_code 😞

However the 404 causes an issue with

maintenance_tasks/test/application_system_test_case.rb

Line 40 in 1519b49

assert_empty page.driver.browser.logs.get(:browser)

since the browser reports navigating to the page as an error. We can work around this with

Suggested change

	assert_title "Not Found"
	assert_title "Not Found"
	assert_includes page.driver.browser.logs.get(:browser).sole.message, "404"