* Make build exit with error code when interrupted

This addresses issue #1493.

Current behavior is that `npm run build` exits code 0 without creating a bundle when interrupted. This change makes the build script catch catchable interruptions and exit with the appropriate error code.

* Better error messages for kill signals

* Don't catch SIGINT

Ctrl+C should exit silently, and already produces a non-zero exit code when sent to the console while `npm run build` is running. Exit code 0 is produced if SIGINT is sent directly to the `node build.js` process, but this is unlikely to happen. A SIGINT handler in `build.js` will also be triggered by Ctrl+C in the console, potentially producing unnecessary noise.

* Style fix

* No changes needed to build.js

Problem is coming from the parent process, `react-scripts`

* Make react-scripts script handle signals

* Clarify context

It is easier to add more scripts later this way.