Add better error message for failed cross-partition requests

As reported in https://github.com/aws/aws-sdk/issues/647 and https://stackoverflow.com/a/54837250, there is a lot of confusion when making requests to GovCloud regions from non-GovCloud regions. The cross-partition requests fail because "[You cannot use IAM credentials from one partition to interact with resources in a different partition](https://docs.aws.amazon.com/whitepapers/latest/aws-fault-isolation-boundaries/partitions.html)." This results in authorization errors that vary across services, for example:


Would the SDK teams be able to a clearer error message for this scenario?