At the moment we are just using the standard 404 message provided by Python's HTTPServer module, which is a bit ugly. We should probably have our own templated page for this.
Closing with WONTFIX - VDI support has been removed (bug 4352).
The error pages shown as a consequence of bug 7851 are not even Python's HTTPServer standard error pages, but the browser's built-in errors. Below are the errors shown in Firefox and Chromium:
Firefox v. 105:
> Secure Connection Failed
> An error occurred during a connection to localhost:300.
> * The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
> * Please contact the website owners to inform them of this problem.
Chromium v. 103:
> This page isn’t working
> localhost didn’t send any data.
The info in comment 3 regards a corner case when an error occurs during sending error data to the browser.
In the regular case, Python's error pages are shown as expected, so bug 7851 would also benefit from solving this bug.