Improve error handling on mega editor endpoint uses

[marcustyphoon]

Concept

make use of #971

idk will figure out what needs to be done when I have two arms

[marcustyphoon]

XKit-Rewritten/src/util/mega_editor.js

Lines 53 to 55 in b75a8ba

	}).then(async response =>
	response.ok ? response.json() : Promise.reject(await response.json())
	),

This doesn't currently give a clean message when the error page is HTML, as it is on at least the HTTP 500 errors given if I break #827.

Changing this would mess with that PR's diff, so I'm not doing it at the moment.