Missing error handling in WebSockets code

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) {