iam:ListAccessKeys denied exception in gathering.py

[j0eblow]

Describe the bug
gathering.py does not handle AccessDenied (e.g. iam:ListAccessKeys is explicitly denied). Here is my quick fix for it (I ran black on it as well).

To Reproduce
Steps to reproduce the behavior, please include information on suspected users/roles that are the source of the issue when possible:

1. Create an IAM role with iam:ListAccessKeys denied (or have an SCP that explicitly denies it).
2. Run pmapper graph create.
3. See error

Expected behavior
The function should handle exceptions. Unless it breaks something.