Stack trace on missing credentials

[rdegraaf]

Describe the bug
If pmapper decides that it needs credentials but cannot find any, it dumps a stack trace.

To Reproduce

1. Make sure that no AWS credentials are present in your shell.

2. pmapper graph create

3. Stack trace:

    Traceback (most recent call last):
      File "/home/rdegraaf/aws-venv/bin/pmapper", line 8, in <module>
        sys.exit(main())
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/principalmapper/__main__.py", line 145, in main
        return graph_cli.process_arguments(parsed_args)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/principalmapper/graphing/graph_cli.py", line 142, in process_arguments
        session = botocore_tools.get_session(parsed_args.profile)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/principalmapper/util/botocore_tools.py", line 45, in get_session
        stsclient.get_caller_identity()  # raises error if it's not workable
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/client.py", line 535, in _api_call
        return self._make_api_call(operation_name, kwargs)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/client.py", line 963, in _make_api_call
        http, parsed_response = self._make_request(
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/client.py", line 986, in _make_request
        return self._endpoint.make_request(operation_model, request_dict)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/endpoint.py", line 119, in make_request
        return self._send_request(request_dict, operation_model)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/endpoint.py", line 198, in _send_request
        request = self.create_request(request_dict, operation_model)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/endpoint.py", line 134, in create_request
        self._event_emitter.emit(
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/hooks.py", line 412, in emit
        return self._emitter.emit(aliased_event_name, **kwargs)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/hooks.py", line 256, in emit
        return self._emit(event_name, kwargs)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/hooks.py", line 239, in _emit
        response = handler(**kwargs)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/signers.py", line 105, in handler
        return self.sign(operation_name, request)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/signers.py", line 189, in sign
        auth.add_auth(request)
      File "/home/rdegraaf/aws-venv/lib/python3.8/site-packages/botocore/auth.py", line 418, in add_auth
        raise NoCredentialsError()
    botocore.exceptions.NoCredentialsError: Unable to locate credentials
   

Expected behavior
A useful error message with no stack trace.