Fix 343 unhandled rejection on connect

[Aschen]

What does this PR do?

Fix #343 unhandled rejection on connect retry.

How should this be manually tested?

• Step 1 : Run this snippet without Kuzzle running:

const {
  Kuzzle,
  Websocket
} = require('./index');

const
  websocketProtocol = new Websocket({ host: 'localhost', port: 7512 }),
  kuzzle = new Kuzzle(websocketProtocol);

  kuzzle.on('networkError', error => {
    console.error(`Network Error: ${error.message}`);
  });

(async () => {
  try {
    await kuzzle.connect();

    console.log(await kuzzle.server.now());
  } catch (error) {
    console.log(error);
  } finally {
    kuzzle.disconnect();
  }
})()

[codecov-io]

Codecov Report

Merging #345 into 6-dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##            6-dev     #345   +/-   ##
=======================================
  Coverage   96.56%   96.56%           
=======================================
  Files          27       27           
  Lines        1397     1397           
=======================================
  Hits         1349     1349           
  Misses         48       48

Impacted Files	Coverage Δ
src/protocols/abstract/realtime.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d14ae29...f05e796. Read the comment docs.