EditPage: Show EditFilterMergedContent hook errors in an errorbox

EditPage: Show EditFilterMergedContent hook errors in an errorbox

Errors that go through the Status object of the EditFilterMergedContent
hook have less control over how their errors are displayed, compared to
the legacy EditFilterMerged hook. Notably, Scribunto wrapped its errors
in an errorbox for increased visibility. That was a good idea in
general, so let's do that for all errors.

Change-Id: I1f0e463841298b8c59ed8bd898c8a4661c5e3aa4