Kill pending errors that are edited by the user

Editing text that contains error overlays does not currently work well. The correspondence between error and source can quickly become lost, and in a number of cases (eg use of transpose-lines) editing can leave empty overlays lying around that are mostly useless and definitely confusing.

There's also a display bug with such empty overlays: because they contain no text, they have no text properties, leading to the `!` in the fringe changing colour. It's pretty confusing and flaky.

I think it is better to simply remove overlays that are edited by the user, so I've implemented that.