in Hybrid Platform UI, error can happen in 2 cases:

• as a response to an AJAX request
• as a server response when generating a full page

AJAX requests (404, 403 and 500):

The app should display an error notification bar at the bottom of the page (same as eZ Platform v1.x). As in eZ Platform v1.x, the error notification bar remains until the user clicks on the closing button.
The content of the error notification bar will be structured like this:
error description (error code): requested URL Copy message

The Copy to clipboard button will have more information than the error displayed.
If it's possible we want to include:

• Name and URL of the page where the error occurred
• AJAX request who returned the error (including the Request Headers and Request Payload)
• Response Headers / Response message

Offline / Slow response

http://www.caniuse.com/#feat=online-status is well supported so we can imagine an overlay that blocks all the actions until the connection to the network is back.
In addition to this feature, we will need also an error notification bar in case of the user loosing the internet connection when the action is executed.
The content of this notification bar can be: "Looks like the server is taking to long to respond. Please try again".

Direct Access

404

The top navigation will be visible. Discovery Bar (left menu) and Action Bar (right menu) are not visible.
We will provide the text to display in the center of the page.
We might include a search form and / or a button to go back to the content structure

403

Same as 404: the top navigation will be visible. Discovery Bar (left menu) and Action Bar (right menu) are not visible.
We will provide the text to display in the center of the page.
We want to include a button Request Access that sends a message to the person in charge of the website. I know that we don't have the AdminEmail as we had in site.ini in legacy. Maybe a great opportunity to had this parameter to a yml file (in siteaccess group and siteaccess).

500

As you said we can't do much more than just displaying a simple page. We'll provide the text and the design.

401

I guess for those two, we should just display the login form.

You are right. For AJAX and direct access, we display the login form.
In case of a direct access, can we keep the URL so the user is redirected to the page he wanted to access?
In a case of an AJAX request, publish for example, we might have an issue if the user is redirected to the publication process. We are loosing the headers if I am right. Any suggestion?