[BUG] Misleading error message if plan has invalid JSON

[mluttrell]

Describe the bug
When the tool is run against a plan with invalid JSON, another exception occurs:

AttributeError: 'Namespace' object has no attribute 'plan'

This is due to this line that appears to reference an invalid/non-existent "plan" parameter: https://github.com/awslabs/terraform-iam-policy-validator/blob/main/iam_check/iam_check.py#L35

To Reproduce
Steps to reproduce the behavior:

1. Run the tool against a file with invalid JSON

Expected behavior
An error message that tells me my JSON is invalid.