webpack: Disable webpack-dev-server overlay for runtime errors.

webpack-dev-server 4.12.0 introduced a global handler that shows a
full-screen overlay for all runtime errors, but it’s redundant with
our blueslip_stacktrace handler and less functional at this time.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

web/webpack.config.ts

@@ -216,6 +216,11 @@ export default (
             }),
         ],
         devServer: {
+            client: {
+                overlay: {
+                    runtimeErrors: false,
+                },
+            },
             devMiddleware: {
                 publicPath: "/webpack/",
                 stats: {