LSP server triggers unhandled rejection when schema endpoint is unavailable

With a config file like:

```
module.exports = {
schema: 'http://localhost:5000/browse/api'
}
```

And the graphql server not actually running, the LSP server will try to connect to the endpoint and trigger promise rejection that is not unhandled:

```
internal/process/warning.js:40 (node:8940) UnhandledPromiseRejectionWarning: FetchError: request to http://localhost:5000/browse/api failed, reason: connect ECONNREFUSED 127.0.0.1:5000
at ClientRequest. (/Users/me/github/graphiql/node_modules/node-fetch/lib/index.js:1455:11)
at ClientRequest.emit (events.js:314:20)
at Socket.socketErrorListener (_http_client.js:428:9)
at Socket.emit (events.js:314:20)
at emitErrorNT (internal/streams/destroy.js:92:8)
at emitErrorAndCloseNT (internal/streams/destroy.js:60:3)
at processTicksAndRejections (internal/process/task_queues.js:84:21)
```

Unhandled rejections are bad as with Node 14 those will just crash the process.