[BUG] sqs_events engine fails hard on non-json if message_format set to json

Description

The sqs_events engine fails hard if configured with sqs.message_format: json if it receives a non-JSON message:

2020-12-08 19:49:26,362 [salt.engines     :137 ][CRITICAL][5016] Engine 'sqs_events' could not be started!
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/salt/engines/__init__.py", line 135, in run
    self.engine[self.fun](**kwargs)
  File "/usr/lib/python3/dist-packages/salt/engines/sqs_events.py", line 189, in start
    _process_queue(
  File "/usr/lib/python3/dist-packages/salt/engines/sqs_events.py", line 162, in _process_queue
    tag=tag, data={"message": salt.utils.json.loads(msg.get_body())}
  File "/usr/lib/python3/dist-packages/salt/utils/json.py", line 97, in loads
    return json_module.loads(s, **kwargs)
  File "/usr/lib/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.8/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Setup

Master config:

some_aws_profile:
  region: us-east-2
  keyid: xxx
  key: yyy

sqs.message_format: json

engines:
  - sqs_events:
      queue: some_queue
      profile: some_aws_profile

Send a plain text message to some_queue using the SQS console (or with the tooling of your choice).

Expected behavior

The engine should not fail with a CRITICAL log and a traceback. A WARNING about an unexpected non-json message feels more appropriate.

Versions Report

Salt Version:
          Salt: 3002.2
 
Dependency Versions:
          cffi: 1.14.4
      cherrypy: Not Installed
      dateutil: 2.7.3
     docker-py: Not Installed
         gitdb: 2.0.6
     gitpython: 3.0.7
        Jinja2: 2.10.1
       libgit2: Not Installed
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 0.6.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     pycparser: 2.20
      pycrypto: 2.6.1
  pycryptodome: 3.6.1
        pygit2: Not Installed
        Python: 3.8.5 (default, Jul 28 2020, 12:59:40)
  python-gnupg: 0.4.5
        PyYAML: 5.3.1
         PyZMQ: 18.1.1
         smmap: 2.0.5
       timelib: Not Installed
       Tornado: 4.5.3
           ZMQ: 4.3.2
 
System Versions:
          dist: ubuntu 20.04 focal
        locale: utf-8
       machine: x86_64
       release: 5.4.0-54-generic
        system: Linux
       version: Ubuntu 20.04 focal

Start in salt/engines/sqs_events.py, especially _process_queue and start, following the traceback shown in the report. Reproduce the problem by sending a plain-text message to an SQS queue configured with sqs.message_format: json. Done means the engine continues running and emits a warning rather than a CRITICAL log with a traceback.