Send all unhandled rejections and uncaught exceptions to the error log

[scinos]

Changes proposed in this Pull Request

• Uses https://nodejs.org/api/process.html#process_event_uncaughtexceptionmonitor to log all unhandled rejections and uncaught exceptions.

Testing instructions

Checkout the branch and add an error, for example add setTimeout( () => fake(), 5000 ); to client/server/index.js

Then start the server with yarn start. You should see something like

09:47:31.146Z ERROR calypso: fake is not defined
    ReferenceError: fake is not defined
        at Timeout._onTimeout (/Users/sergio/src/automattic/wp-calypso/build/webpack:/client/server/index.js:83:2)
        at listOnTimeout (internal/timers.js:549:17)
        at processTimers (internal/timers.js:492:7)

(the timestamp and the string ERROR calypso: implies it comes from bunyan's error logger)

[matticbot]

Test live: https://calypso.live/?branch=add/generic-error-logger

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[griffbrad]

👍🏻 LGTM