TypeError: ctx.onerror is not a function
Environment
- Nodejs: 8.11.1
- Koa: 2.5.1

```
const onerror = err => ctx.onerror(err);
                               ^

TypeError: ctx.onerror is not a function
    at Array.onerror (./server/node_modules/koa/lib/application.js:147:32)
    at listener (./server/node_modules/on-finished/index.js:169:15)
    at onFinish (./server/node_modules/on-finished/index.js:100:5)
    at callback (./dist/server/node_modules/ee-first/index.js:55:10)
    at ServerResponse.onevent (./server/node_modules/ee-first/index.js:93:5)
    at emitNone (events.js:111:20)
    at ServerResponse.emit (events.js:208:7)
    at onFinish (_http_outgoing.js:720:10)
    at _combinedTickCallback (internal/process/next_tick.js:131:7)
    at process._tickCallback (internal/process/next_tick.js:180:9)
```

How to fix it?
- Replace: `const onerror = err => ctx.onerror(err);`
- With: `const onerror = ctx.onerror;`

~~*paragraph removed by fl0w*~~
