WebSocket: fix error object on connection error

[scottinet]

Description

On a websocket connection error, the error thrown by the SDK has the following message: [object Object]

This is because on a connection error, ws sends a complex object, but our error handler processes it as if it's a string.

How to test it

Run this snippet:

const {Kuzzle, WebSocket} = require('kuzzle-sdk');
const
  ws = new WebSocket('localhost'),
  kuzzle = new Kuzzle(ws);

kuzzle.connect().catch(err => console.error(err));
kuzzle.disconnect();

Before this PR, this prints:

Error: [object Object]
    ... stack trace ...

With this PR:

Error: WebSocket was closed before the connection was established
    ... stack trace ...

[codecov-io]

Codecov Report

Merging #373 into 6-dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           6-dev     #373      +/-   ##
=========================================
+ Coverage   96.7%   96.71%   +<.01%     
=========================================
  Files         29       29              
  Lines       1427     1429       +2     
=========================================
+ Hits        1380     1382       +2     
  Misses        47       47

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

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[Aschen]

Don't we loose the stacktrace returned by Kuzzle?

[Aschen]

It will be fixed with https://jira.kaliop.net/browse/KZL-1001, nevermind

[scottinet]

Well, even if it will be fixed, I think your point is valid. I prefer to fix this now, even if it is redundant with a future task.

[scottinet]

@Aschen > fixed