provider/aws: Prevent crash on account ID validation

[radeksimko]

Fixes #8729

Introduced in #8638

Test plan

provider "aws" {
  region = "us-west-2"
  allowed_account_ids = ["invalid"]
}

expected output:

Error refreshing state: 1 error(s) occurred:

* Account ID not allowed (123456789012)

[stack72]

Nice catch here @radeksimko :)

This LGTM!

[ghost]

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