Makes the script crash on unhandled rejections instead of silently ig…

…noring them

See jestjs/jest#5311 (comment)

✗ Errored » Asynchronous Error
      in Integration tests
      in test/integration-test.js
/home/macbre/github/phantomas/test/integration-test.js:19
	throw err;
	^

Error: Protocol error (Performance.getMetrics): Session closed. Most likely the page has been closed.
    at CDPSession.send (/home/macbre/github/phantomas/node_modules/puppeteer/lib/cjs/puppeteer/common/Connection.js:195:35)
    at Page.metrics (/home/macbre/github/phantomas/node_modules/puppeteer/lib/cjs/puppeteer/common/Page.js:699:45)
    at /home/macbre/github/phantomas/lib/browser.js:266:37
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)

test/integration-test.js

@@ -12,6 +12,13 @@ var vows = require('vows'),
 
 var WEBROOT = 'http://127.0.0.1:8888'; // see start-server.sh
 
+// Makes the script crash on unhandled rejections instead of silently
+// ignoring them. In the future, promise rejections that are not handled will
+// terminate the Node.js process with a non-zero exit code.
+process.on('unhandledRejection', err => {
+	throw err;
+});
+
 // run the test
 var suite = vows.describe('Integration tests').addBatch({
 	'test server': {