Backport (7.8) - Channel manager / template composer problem when site returns 404

I am not 100% sure, but I think the template composer spinner does not go away when the site returns with a 404. Also, the template composer is then unusable.

This is particularly annoying when you by accident broke for example the 'home page' by changing the relative content path (through url management in hst-config-editor ) of the sitemap item for the homepage to a non existing document returning in a 404 : When you are in that situation, you cannot do anything any more, even not go to the url management because hst config editor is not accessible.

You can easily reproduce this with the archetype created site. go to channel mngr, go the config editor, go to url management and change rel content path of hst:root sitemap item to non existing doc. Now, you cannot do anything any more for that channel. Even not fix it. Only the console helps out now

Alternative reproduction steps:
1. Edit document1, and create an internal link to document2
2. Delete the document2
3. View document1 in the Channel Manager
4. Click the link created in step 1. The error page shows.
5. Click "Edit Channel"
6. Click "Save & Close"
7. Click "Edit Channel"

Expected result:

• After step 6, the channel is indicated as being changed: "A previous version is live"
• After step 7, the page is opened in edit mode

Actual result:

• After step 6, the channel manager looks the same as after step 4
• After step 7, the spinners keep blocking the function of the Template Composer without any message