[Bug][Chrome][Randomly] Error: not opened

Hello All,

We found the following error appearing randomly during our test ( we have 300 links need to be tested).
And we checked the captured page, they are okay.
Appear: 5 times

```
Error: not opened
at WebSocket.send (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/WebSocket.js:356:18)
at Chrome.enqueueCommand (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:115:16)
at /usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:79:28
at new Promise ()
at Chrome.send (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:78:16)
at Object.handler [as handleCertificateError] (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/api.js:32:23)
at Chrome. (/usr/local/lib/node_modules/backstopjs/node_modules/chromy/dist/index.js:477:40)
at emitOne (events.js:116:13)
at Chrome.emit (events.js:211:7)
at Chrome.handleMessage (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:285:16)
```

**Backstop: 3.0.38**
Node: 8.9.4
NPM: 5.6.0
OS: Centos
Thank you,
Ken