Add additional information for postcss errors (#6282)

Created by: buildbreakdo

Fixes #6282 (closed): If build compiler error has postcssNode property, add additional information to the error message. Default compiler CSS error messages are opaque. To verify:

npx create-react-app css-test
cd css-test
echo "label[for=*] { margin-right: 12px; }" > ./src/App.css
yarn build

Outputs error:

Replace css-test/node_modules/react-scripts/build.js lines:

messages = formatWebpackMessages({
  errors: [err.message],
  warnings: [],
});

With :

        let errMessage = err.message;

        // Add additional information for postcss errors
        if (err.hasOwnProperty('postcssNode')) {
          errMessage += '\n' + path.basename(
            err['postcssNode'].source.input.file
          ) + '\nCompileError: Begins at selector ' +
            err['postcssNode'].selector +' (' +
            err['postcssNode'].source.start.line + ':' +
            err['postcssNode'].source.start.column +
            ')';
        }

        messages = formatWebpackMessages({
          errors: [errMessage],
          warnings: [],
        });

Run yarn build to view new compiler error output: