Catch non-JSON payloads
Catch non-JSON payloads

The lambda function currently dies if the payload is not JSON.  I discovered this when sending to a subscribed topic using the
AWS cli:

```
aws sns publish --topic-arn arn:aws:sns:eu-west-2:xxxx:yyyy --message "${msg}"
```

From cloudwatch logs:
```
No JSON object could be decoded: ValueError
Traceback (most recent call last):
File "/var/task/lambda_function.py", line 122, in lambda_handler
json_msg = json.loads(sns['Message'])
File "/usr/lib64/python2.7/json/__init__.py", line 339, in loads
return _default_decoder.decode(s)
File "/usr/lib64/python2.7/json/decoder.py", line 364, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib64/python2.7/json/decoder.py", line 382, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
```

This commit traps the parse error and creates an empty hash
in `json_msg`.  The rest of the function continues normally
and the message is processed and received by Slack correctly.

builtinnya/aws-sns-slack-terraform,builtinnya/aws-sns-slack-terraform