Allow throwing an error inside route handler when `continueOnError` is true
Hi!
came from #97 , created a sandbox that reproduces the problem -> [sandbox](https://codesandbox.io/s/new-cdn-37y4w).
note the difference between sending a request with correct type but invalid body and a request with wrong type.

it happens when you use a [centralized error handling](https://github.com/koajs/koa/wiki/Error-Handling) middleware and you throw a proper error in route handlers when `ctx.invalid` is set.

I think it should be supported and work like a charm so I did some brain work and came up with a solution, will open a pr for this.