When asked to display an error URL, WebEnginePart displayed another
error telling that the web page "error:/..." can't be reached.
To avoid this, add a scheme handler for the "error" scheme to the default
profile. The handler replies with the html code read from an error.html page
with placeholders replaced by the error details contained in the URL.
Note: currently this doesn't affect the error message displayed when the URL is
valid but points to a non-existent page: in this case, an error message from
QWebEngine itself is displayed.
The code used to produce the error page is taken, with small changes, by
webpage.cpp from KWebKitPart. The error page, too is taken from KWebKitPart and
modified a little.