Public API doesn't work in error.hbs template

Issue Summary

Public API doesn’t work in error.hbs template

Steps to Reproduce

1. I had tried use this code for testing error page with enabled api

$.get(ghost.url.api('posts', {limit: "all", include: "tags"})).done(function(data){
      searchData = data.posts;
      console.log(searchData);
 })

and I get this message:

ghost is not defined

Technical details:

• Ghost Version: 0.8.0
• Node Version: 0.12.7
• Browser/OS: chrome
• Database: mysql

[ErisDS]

I’ve always considered that it is bad practice to use the main layout on the error page, and more specifically to include ghost_head or foot on the error template. I think it’s better to have a 500 error page that can render freely of any other aspect of the software, to minimise the chance of causing the error to be thrown again.

The server-side error environment is quite different to rendering a normal page of your theme, lots of things aren’t loaded, and the reason for this particular issue is that if you call ghost_head from an error page it simply exits rather than adding anything, so as not to cause any more errors…

The default error template provided by Ghost is entirely stand-alone as an example of what we consider best practice here. You can see this file here: user-error.hbs.

However, I’ve seen that using the default layout in the error template is common amongst themes, and I have a feeling the reason for that is symptom of a different problem - the fact that error.hbs is used for both 404s as well as 5xx errors. It’s pretty common to want to heavily customise a 404 page and render all sorts of useful stuff (such as search) on that page to get a user to stay on the site, meanwhile I think it’s fairly standard to have a very simple 5xx template.

I’d propose extending the error handling system to support error-404.hbs and error-500.hbs, to handle different sorts of errors differently. I’d propose that ghost_head should still not output anything if we’re on a 500 or 5xx error, but that it would be ok to output some things (such as the API scripts) if we’re on a 404 page?

[ErisDS]

Closing in favour of the feature spec: #8079