Server crash when requesting a non-existant file in /_ream
To reproduce just clone this project: https://github.com/jazoom/ream-minimal
then run:
$ yarn
$ npm run build
$ npm run start
then navigate to `http://localhost:4000/_ream/not-exist.js`

You will get this error:

```
TypeError: next is not a function
    at SendStream.error (C:\WebProjects\ream-minimal\node_modules\serve-static\index.js:121:7)
    at emitOne (events.js:96:13)
    at SendStream.emit (events.js:188:7)
    at SendStream.error (C:\WebProjects\ream-minimal\node_modules\send\index.js:282:17)
    at SendStream.onStatError (C:\WebProjects\ream-minimal\node_modules\send\index.js:433:12)
    at onstat (C:\WebProjects\ream-minimal\node_modules\send\index.js:734:26)
    at FSReqWrap.oncomplete (fs.js:123:15)
```