What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
No. It's hard to write test and maybe it is not necessary.

Summary

1. According to Node.js document, it is not correct to write something like

console.log('hello, world');
process.exit()

https://nodejs.org/dist/latest-v9.x/docs/api/process.html#process_process_exit_code

Rather than calling process.exit() directly, the code should set the process.exitCode and allow the process to exit naturally by avoiding scheduling any additional work for the event loop:

2. There is an issue sass worker hangs on js compilation errors under certain circumstance sass-loader#523 fixed by this pull request.

The issue is that Node.js doesn't exit itself after the process.exit(1) in line 367 of webpack.js.
Some callbacks of fs.stat and fs.readFile were never called at this time.

I am not familiar with Node.js and libuv. The following is my guess.
webpack calls fs functions many times, and they will use the thread pool of libuv.
node-sass will use the same thread pool to do its work.
After the call to process.exit(), the fs functions will fail silently and won't release their threads, but node-sass is waiting for threads and process.exit() is waiting for node-sass.

Does this PR introduce a breaking change?
No.