[BUG] salt-minion error when master not available

[twangboy]

Description
If the master is not available the minion will throw a stacktrace when starting.

This was first discovered on Windows but doesn't seem to be specific to Windows. The stacktrace is also observed on Ubuntu 20.04.

[DEBUG   ] SaltEvent PUB socket URI: /var/run/salt/minion/minion_event_67365db408_pub.ipc
[DEBUG   ] SaltEvent PULL socket URI: /var/run/salt/minion/minion_event_67365db408_pull.ipc
[DEBUG   ] salt.crypt.get_rsa_pub_key: Loading public key
[DEBUG   ] Closing AsyncReqChannel instance
[ERROR   ] Exception in callback functools.partial(<function wrap.<locals>.null_wrapper at 0x7f0dd3c64790>, <salt.ext.tornado.concurrent.Future object at 0x7f0dd3c68970>)
Traceback (most recent call last):
  File "/home/shane/src/salt/salt/ext/tornado/ioloop.py", line 606, in _run_callback
    ret = callback()
  File "/home/shane/src/salt/salt/ext/tornado/stack_context.py", line 278, in null_wrapper
    return fn(*args, **kwargs)
  File "/home/shane/src/salt/salt/ext/tornado/ioloop.py", line 628, in _discard_future_result
    future.result()
  File "/home/shane/src/salt/salt/ext/tornado/concurrent.py", line 249, in result
    raise_exc_info(self._exc_info)
  File "<string>", line 4, in raise_exc_info
  File "/home/shane/src/salt/salt/ext/tornado/gen.py", line 1064, in run
    yielded = self.gen.throw(*exc_info)
  File "/home/shane/src/salt/salt/crypt.py", line 653, in _authenticate
    creds = yield self.sign_in(channel=channel)
  File "/home/shane/src/salt/salt/ext/tornado/gen.py", line 1056, in run
    value = future.result()
  File "/home/shane/src/salt/salt/ext/tornado/concurrent.py", line 249, in result
    raise_exc_info(self._exc_info)
  File "<string>", line 4, in raise_exc_info
  File "/home/shane/src/salt/salt/ext/tornado/gen.py", line 1070, in run
    yielded = self.gen.send(value)
  File "/home/shane/src/salt/salt/crypt.py", line 779, in sign_in
    ret = self.handle_signin_response(sign_in_payload, payload)
  File "/home/shane/src/salt/salt/crypt.py", line 791, in handle_signin_response
    clear_signature = payload["sig"]
KeyError: 'sig'
[ERROR   ] Minion unable to successfully connect to a Salt Master.

Setup
Install Salt version 3005+

• on-prem machine
• VM (Virtualbox, KVM, etc. please specify)
• VM running on a cloud service, please be explicit and add details
• container (Kubernetes, Docker, containerd, etc. please specify)
• or a combination, please be explicit
• jails if it is FreeBSD
• classic packaging
• onedir packaging
• used bootstrap to install

Steps to Reproduce the behavior
Start the salt-minion in debug mode:

salt-minion -l debug

Expected behavior
It should just fail to connect and keep trying indefinitely, at some randomized interval.

Versions Report

salt --versions-report

(Provided by running salt --versions-report. Please also mention any differences in master/minion versions.)

Salt Version:
          Salt: 3005

Dependency Versions:
          cffi: Not Installed
      cherrypy: Not Installed
      dateutil: Not Installed
     docker-py: Not Installed
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 3.1.2
       libgit2: Not Installed
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.0.4
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     pycparser: Not Installed
      pycrypto: Not Installed
  pycryptodome: 3.15.0
        pygit2: Not Installed
        Python: 3.8.10 (default, Jun 22 2022, 20:18:18)
  python-gnupg: Not Installed
        PyYAML: 6.0
         PyZMQ: 20.0.0
         smmap: Not Installed
       timelib: Not Installed
       Tornado: 4.5.3
           ZMQ: 4.3.3

System Versions:
          dist: ubuntu 20.04 Focal Fossa
        locale: utf-8
       machine: x86_64
       release: 5.4.0-124-generic
        system: Linux
       version: Ubuntu 20.04 Focal Fossa```
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