upgrade-zulip-from-git: Improve webpack failure error handling.

We've had a number of unhappy reports of upgrades failing due to
webpack requiring too much memory.  While the previous commit will
likely fix this issue for everyone, it's worth improving the error
message for failures here.

We avoid doing the stop+retry ourselves, because that could cause an
outage in a production system if webpack fails for another reason.

Fixes #20105.

scripts/lib/upgrade-zulip-stage-2

@@ -215,7 +215,15 @@ elif args.from_git:
     # update-prod-static with the Git upgrade process.  But it'll fail
     # safely; this seems like a worthwhile tradeoff to minimize downtime.
     logging.info("Building static assets...")
+    try:
         subprocess.check_call(["./tools/update-prod-static"], preexec_fn=su_to_zulip)
+    except subprocess.CalledProcessError:
+        logging.error("Failed to build static assets.")
+        if IS_SERVER_UP:
+            logging.error("Usually the cause is insufficient free RAM to run webpack.")
+            logging.error("Try stopping the Zulip server (scripts/stop-server) and trying again.")
+        sys.exit(1)
+
     logging.info("Caching Zulip Git version...")
     subprocess.check_call(["./tools/cache-zulip-git-version"], preexec_fn=su_to_zulip)
 else: