scripts: Only stop/restart zulip_deliver_scheduled_* processes if known.

Running `supervisorctl stop` or `supervisorctl restart` on a process
name which is not known is an error:

```
$ supervisorctl stop nonexistent-process
nonexistent-process: ERROR (no such process)
$ echo $?
1
```

ef6d0ec5ca moved
zulip_deliver_scheduled_* out of the `workers:` group.  Since upgrades
run `stop-server` before applying puppet, the list of processes at
that time is from the previous version of Zulip, so may not have the
new `zulip_deliver_scheduled_*` names -- and the `stop-server` will
hence fail.

If the upgrade is not applying puppet, it will `restart-server`. At
that point, the old names will still be in the configuration, so
relying on the current `superisorctl status` is the best gauge of what
exists to restart.

In short, only ever stop/start/restart the `zulip_deliver_scheduled_*`
processes if `supervisorctl status` knows about them already.

scripts/restart-server

@@ -80,11 +80,16 @@ if has_application_server():
         workers.extend(list_supervisor_processes("zulip-workers:*"))
 
     if has_application_server(once=True):
+        # These used to be included in "zulip-workers:*"; since we may
+        # be restarting an older version of Zulip, which has not
+        # applied puppet to reload the new list of processes, only
+        # stop them if they currently exist according to
+        # `supervisorctl`.
         workers.extend(
-            [
+            list_supervisor_processes(
                 "zulip_deliver_scheduled_emails",
                 "zulip_deliver_scheduled_messages",
-            ]
+            )
         )
 
 if has_process_fts_updates():

scripts/stop-server

@@ -13,6 +13,7 @@ from scripts.lib.zulip_tools import (
     WARNING,
     has_application_server,
     has_process_fts_updates,
+    list_supervisor_processes,
 )
 
 deploy_path = os.path.realpath(os.path.join(os.path.dirname(__file__), ".."))
@@ -41,11 +42,15 @@ if has_application_server():
     services.extend(["zulip-tornado", "zulip-tornado:*"])
     services.append("zulip-workers:*")
     if has_application_server(once=True):
+        # These used to be included in "zulip-workers:*"; since we may
+        # be stopping an older version of Zulip, which has not applied
+        # puppet to reload the new list of processes, only stop them
+        # if they currently exist according to `supervisorctl`.
         services.extend(
-            [
+            list_supervisor_processes(
                 "zulip_deliver_scheduled_emails",
                 "zulip_deliver_scheduled_messages",
-            ]
+            ),
         )
 
 subprocess.check_call(["supervisorctl", "stop", *services])