No/bad AJAX response must be handled

[mvorisek]

When AJAX response is not returned (timeout) or wrong (non-200 status code or wrong JSON/JS syntax)

then the response must be handled in Atk4 frontend JS
and the original AJAX request must be canceled (when not-finished yet, like timeout)

demos/_unit-test/late-output-error.php can be used for Behat testing of wrong AJAX responses

[mvorisek]

another example is atk4/login@f475d95 and atk4/login/demos/admin-roles.php demo

when

is clicked and then

is clicked, faded background is the only thing displayed, no error message, nothing, in network view the following shown:

for normal users, it is nearly impossible to tell the problem

[mvorisek]

another easy steps to reproduce/modal usecase described in #1883 (comment)

[mvorisek]

mostly fixed by #1925 and #1919