Avoid emitting error if upload was not done on closing request
I noticed that when a request ends, if at least one of the uploaded file streams was not consumed, [an error is emitted](https://github.com/jaydenseric/apollo-upload-server/blob/master/src/middleware.mjs#L152). I've found out the hard way, that if you don't subscribe to the `'error'` event on the readable stream, the emission halts the node process and brings down the entire server.

```sh
FileStreamDisconnectUploadError: Request disconnected during file upload stream parsing.
    at IncomingMessage.request.on (/dev/project/node_modules/apollo-upload-server/lib/middleware.js:151:15)
    at IncomingMessage.emit (events.js:182:13)
    at resOnFinish (_http_server.js:564:7)
    at ServerResponse.emit (events.js:187:15)
    at onFinish (_http_outgoing.js:683:10)
    at process._tickCallback (internal/process/next_tick.js:61:11)
Emitted 'error' event at:
    at IncomingMessage.request.on (/dev/project/node_modules/apollo-upload-server/lib/middleware.js:149:32)
    at IncomingMessage.emit (events.js:182:13)
    [... lines matching original stack trace ...]
    at process._tickCallback (internal/process/next_tick.js:61:11)
```

This has the nasty side-effect of potentially allowing **any GraphQL client to send a file under an unexpected field and tear down your server**. I realize this after tracing a bug to the UI where the `file: Upload` field in the mutation was inadvertently being sent as `attachment`. The API server was dying, without any chance of recovering or catching the error.

You can try this using the `apollo-upload-examples`. Just change the [expected `file` field in the `singleUpload` mutation](https://github.com/jaydenseric/apollo-upload-examples/blob/master/api/resolvers.mjs#L54) to any other name on the UI and run the upload.

Is there any way of preventing this kind of behavior?