docs: Update error-handling.md (#1239)
docs: Update error-handling.md (#1239)

Docs incorrectly states that exposed errors are being logged by default error listener:
"which simply log the error if error.expose is true"
https://github.com/koajs/koa/blob/master/lib/application.js#L185
fl0w/koa,nswbmw/koa,fl0w/koa,koajs/koa