Hi Mark,
If there can be an error we need to be prepared to deal with it.
For designing a proper solution I need some more information. Which kinds of errors can be produced and at which point of the workflow? Is there a way to solve them without user intervention? Is there some information we can provide to help the user to solve them? Will retrying later be a likely way to solve the issue?
From the process of disabling, the only error point I can imagine is clicking on the "disable/enable" button and the action not taking effect because the place where we store such info fails to do so (I don't know if that is an external server). For this kind of error, we can just inform using a standard MediaWiki notification bubble with some message along the lines of "It was not possible to disable Media Viewer due to some technical difficulties. Please try again later." and keep the panel opened so that the user can try again. In any case, we should not show the confirmation popup if the action had no effect. In any case, I'm making here some assumptions since I don't know the internals of the process.
Pau