Instead of looking for a fixed error string, it uses error codes from aws cli
Previos condition was not catching this error message:
An error occurred (ExpiredToken) when calling the GetCallerIdentity operation: The security token included in the request is expired
Also forced the output of the command to json. In some tests I was doing was failing becuase it was sending output as text
json-asff mode outputs JSON, similar to the standard 'json' mode with one check per line, but in AWS Security Finding Format - used by AWS Security Hub
Currently uses a generic Type, Resources and ProductArn value, but sets the Id to a unique value that includes the details of the message, in order to separate out checks that run against multiple resources and output one result per resource per check. This ensures that findings can be updated, should the resource move in or out of compliance
securityhub mode generates the ASFF JSON and then passes it to an 'aws securityhub batch-import-findings' call, once per resource per check. Output to the screen is similar to the standard mode, but prints whether or not the finding was submitted successfully
Fixes#524
