provider/aws: Fail silently in ValidateCredentials for IAM users

[catsby]

WIP, don't merge yet

This should help fix #2828 and #2955 (introduced by #2730) by failing silently in the event of an AccessDenied or ValidationError error message from the iam.GetUser call. Instance Profiles may not have this role/permission, but right now we're failing here. If there exists any hierarchy of nodes (dependencies), you can trigger a crash (patched in #2963).

• If a role has no IAM policy attached, any authentication via iam.GetUser with an IAM profile will receive an AccessDenied error message.
• If the role does have an IAM policy, any authentication via iam.GetUser will return a ValidationError regarding a missing user name

Testing this now, may change to specifically confirm the awsErr.Message() contains iam:GetUser message Good To Go

[phinze]

LGTM

[ghost]

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