Remove two blank error log statements in the default error handler
The default error handler calls `console.error()` (with no arguments) twice. 
https://github.com/koajs/koa/blob/9ee65843d9be96329a3279c63657c2970e260acf/lib/application.js#L204

Unfortunately this causes some confusing noise in AWS Lambda + CloudWatch which shows these as "ERROR undefined" log entries

Would you consider a PR to do one of the following:
- simply remove these two error statements
- remove these two empty statements AND add two `\n` inside the call `console.error(msg.replace(/^/gm, '  '));`
- add an option to skip the log statements
 
Thanks