Handle 'Not Found' errors to allow 'loaded' event

[laurenancona]

We've been running our own fork with this patch since 0.11.0 because the errors prevent loaded from returning true. Still results in noisy logs, but find it preferable to silent failure while debugging.

Closes #1800.

[lucaswoj]

Thank you for the PR @laurenancona!

Next Steps

• create an ajax.Error error class with statusText and statusCode properties (requires changing both util/ajax and util/browser/ajax)
• check for error instanceof ajax.Error && error.statusCode === 404 before silently dismissing the error
• squash into one commit

[laurenancona]

Excellent, thanks @lucaswoj. We're taking a shot at it. 🔫

[lucaswoj]

Thank you @laurenancona! Ping me in Slack if you have any questions!

[lucaswoj]

Replaced by #2176