Bug/Improvement: Fetch-with-error-handling should get json from Flows error-handler

[mhsdesign]

When there is an exception in a Ui ajax request, Flows error-handler doesnt respect the accept header, and we dont set it.

Accept: application/json

For better display and further handling, we need errors in the json format.

Maybe the proposed header Accept: application/json is not always correct, as i know of some cases where we fetch xml from the server, for example in the link editor.

Currently, the "fetchWithErrorHandling" helper tries to put the Html from the Flow exception-handler into a temporary dom element to retrieve the innerText and tries to display the headline or something, but this is rather suboptimal and not always working correctly.

[mhsdesign]

This is the respective ancient flow issue :D neos/flow-development-collection#550