MaxFileSizeUploadError can't be caught and crashes the server
Hi!

I have a problem with a custom middleware and the use of the processRequest function.
Here is my custom middleware:

```
try {
    return processRequest(request, options)
                    .then(body => {
                        console.log("body is : ", body);
                        ctx.request.body = body;
                        return next();
                    })
                    .catch(error => {
                        console.log("Error catched : ", error);
                        if (error.status && error.expose) response.status(error.status);
                        return next(error)
                    });
} catch (error) {
    console.log("Error catched bis : ", error);
}
```

If I try to upload a file bigger than the allowed limit, I get the following error and my app crash.

```
events.js:137
      throw er; // Unhandled 'error' event
      ^

MaxFileSizeUploadError: File truncated as it exceeds the size limit.
    at FileStream.file.stream.once (/Volumes/lake/projects/evibe/api-lovejs/node_modules/apollo-upload-server/lib/middleware.js:35:13)
    at Object.onceWrapper (events.js:255:19)
    at FileStream.emit (events.js:160:13)
```

I can't capture the error.
Any idea? 

Thanks :)