webpack: Only show errors and warnings

Otherwise there is just too much noise and when something is broken, the
error is above 3 pages of pointless logs.

See https://webpack.js.org/configuration/stats/

Cherry-picked from cockpit-podman commit 5989b20a05a.

webpack.config.js

@@ -79,6 +79,7 @@ module.exports = {
     // cockpit.js gets included via <script>, everything else should be bundled
     externals: { "cockpit": "cockpit" },
     devtool: "source-map",
+    stats: "errors-warnings",
 
     optimization: {
         minimize: production,