build: ensure JS tests fail when the Karma Webpack build fails

Adds a "DieHardPlugin" to the Webpack build. See enclosed code comment
for more details.

webpack.common.config.js

@@ -26,6 +26,27 @@ var defineFooter = new RegExp('(' + defineCallFooter.source + ')|('
 var staticRootLms = process.env.STATIC_ROOT_LMS || './test_root/staticfiles';
 var staticRootCms = process.env.STATIC_ROOT_CMS || (staticRootLms + '/studio');
 
+class DieHardPlugin {
+  /* A small plugin which ensures that if Webpack fails, it causes the surrounding process to fail
+   * as well. This helps us prevent JavaScript CI from "false passing" upon build failures--that is,
+   * we want to avoid having another situation where the Webpack build breaks under Karma (our
+   * test runner) but Karma just lets it slide and moves on to the next test suite.
+   *
+   * One would imagine that this would be Webpack's default behavior (and maybe it is?) but,
+   * regardless, karma-webpack does not seem to consider Webpack build failures to be fatal errors
+   * without this plugin. We don't fully understand it, but this is good enough given that we plan
+   * to remove all JS in this repo soon (https://github.com/openedx/edx-platform/issues/31620).
+   *
+   * Inpsired by: https://github.com/codymikol/karma-webpack/issues/49#issuecomment-842682050
+   */
+  apply(compiler) {
+    compiler.hooks.failed.tap('DieHardPlugin', (error) => {
+      console.error(error);
+      process.exit(1);
+    });
+  }
+}
+
 var workerConfig = function() {
     try {
         return {
@@ -153,6 +174,7 @@ module.exports = Merge.smart({
                 // any other way to declare that dependency.
                 $script: 'scriptjs'
             }),
+            new DieHardPlugin(),
         ],
 
         module: {