The code for sending requests via WebSockets misses proper error handling, such that problems cannot be handled in user code.

#### How to reproduce
* Should be reproducible on any PC that runs Node.js
* Disconnect from the Internet
* Run the following code with Node.js

```
const steem = require("steem");
steem.api.setOptions({ url: "wss://steemd.privex.io" });
steem.api.getAccounts(["nafestw"], (err, resp) => {
  console.log("Callback is called. err = " + err);
});
```

#### Expected behavior
The callback is called with `err` set to an object describing the error, which should be printed to the console.

#### Actual behavior
The following error occurs and the callback function is **not** called.
```
Unhandled rejection Error: getaddrinfo ENOTFOUND steemd.privex.io steemd.privex.io:443
    at errnoException (dns.js:55:10)
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:97:26)
```

#### Suggested Fix
Adding the missing `.catch` in `src/api/transports/ws.js` should fix the problem:
```
diff --git a/src/api/transports/ws.js b/src/api/transports/ws.js
index 863ec4f..814d701 100644
--- a/src/api/transports/ws.js
+++ b/src/api/transports/ws.js
@@ -94,7 +94,7 @@ export default class WsTransport extends Transport {
       this._requests.set(_request.message.id, _request);
       this.ws.send(JSON.stringify(_request.message));
       return deferral;
-    });
+    }).catch(err => callback(err, null));
   }

   onError(error) {
```

<br /><hr/><em>Posted on <a href="https://utopian.io/utopian-io/@nafestw/missing-error-handling-in-websockets-code">Utopian.io -  Rewarding Open Source Contributors</a></em><hr/>