supervisor: Retry, with backoff, to connect to supervisor socket.

If `zulip-puppet-apply` is run during an upgrade, it will immediately
try to re-`stop-server` before running migrations; if the last step in
the puppet application was to restart `supervisor`, it may not be
listening on its UNIX socket yet.  In such cases, `socket.connect()`
throws a `FileNotFoundError`:

```
Traceback (most recent call last):
  File "./scripts/stop-server", line 53, in <module>
    services = list_supervisor_processes(services, only_running=True)
  File "./scripts/lib/supervisor.py", line 34, in list_supervisor_processes
    processes = rpc().supervisor.getAllProcessInfo()
  File "/usr/lib/python3.9/xmlrpc/client.py", line 1116, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python3.9/xmlrpc/client.py", line 1456, in __request
    response = self.__transport.request(
  File "/usr/lib/python3.9/xmlrpc/client.py", line 1160, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python3.9/xmlrpc/client.py", line 1172, in single_request
    http_conn = self.send_request(host, handler, request_body, verbose)
  File "/usr/lib/python3.9/xmlrpc/client.py", line 1285, in send_request
    self.send_content(connection, request_body)
  File "/usr/lib/python3.9/xmlrpc/client.py", line 1315, in send_content
    connection.endheaders(request_body)
  File "/usr/lib/python3.9/http/client.py", line 1250, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/usr/lib/python3.9/http/client.py", line 1010, in _send_output
    self.send(msg)
  File "/usr/lib/python3.9/http/client.py", line 950, in send
    self.connect()
  File "./scripts/lib/supervisor.py", line 10, in connect
    self.sock.connect(self.host)
FileNotFoundError: [Errno 2] No such file or directory
```

Catch the `FileNotFoundError` and retry twice more, with backoff.  If
it fails repeatedly, point to `service supervisor status` for further
debugging, as `FileNotFoundError` is rather misleading -- the file
exists, it simply is not accepting connections.

scripts/lib/supervisor.py

@@ -1,4 +1,5 @@
 import socket
+import time
 from http.client import HTTPConnection
 from typing import Dict, List, Optional, Tuple, Union
 from xmlrpc import client
@@ -7,7 +8,19 @@ from xmlrpc import client
 class UnixStreamHTTPConnection(HTTPConnection):
     def connect(self) -> None:
         self.sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
-        self.sock.connect(self.host)
+        connected = False
+        for i in range(0, 2):
+            try:
+                self.sock.connect(self.host)
+                connected = True
+                break
+            except FileNotFoundError:
+                # Backoff and retry
+                time.sleep(2**i)
+        if not connected:
+            raise Exception(
+                "Failed to connect to supervisor -- check that it is running, by running 'service supervisor status'"
+            )
 
 
 class UnixStreamTransport(client.Transport):