why context.onerror remove headers?

[menems]

https://github.com/koajs/koa/blob/v2.x/lib/context.js#L121

I don't understand why headers are removed if you throw an error?
I'm sure there is a reason.

concrete example: if I use cors and jwt middleware. domain is authorize to make a request but try to access a unauthorize api, why Access-control...headers should be removed?

The only way is for now, to catch the error(401) and set ctx.status = 401.

But if i want to centralize my errors, i 'd like to use the app.on('error') for example and not add before all other middeleware something like that:

app.use(ctx => {
   return next().catch(e => ctx.status = e.status)
}

[tj]

dup, convo is over at #571, not really a consensus yet but it's definitely awkward