* 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
