Make build exit with error code when interrupted (facebook#1496)

* Make build exit with error code when interrupted

This addresses issue facebook#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

packages/react-scripts/bin/react-scripts.js

@@ -13,6 +13,22 @@ case 'test':
     [require.resolve('../scripts/' + script)].concat(args),
     {stdio: 'inherit'}
   );
+  if (result.signal) {
+    if (result.signal == 'SIGKILL') {
+      console.log(
+        'The build failed because the process exited too early. ' +
+        'This probably means the system ran out of memory or someone called ' +
+        '`kill -9` on the process.'
+      );
+    } else if (result.signal == 'SIGTERM') {
+      console.log(
+        'The build failed because the process exited too early. ' +
+        'Someone might have called `kill` or `killall`, or the system could ' +
+        'be shutting down.'
+      );
+    }
+    process.exit(1);
+  }
   process.exit(result.status);
   break;
 default: