Fix a small bug where we would allow duplicates when tagging a SNS resource.
I stumbled onto another issue: when snapshot-matching the Exception raised by duplicated key, a message field would be added to the body of the response with Localstack, which is not present in AWS.
Example:

      "duplicate-key-error": {
        "Error": {
          "Type": "Sender",
          "Code": "InvalidParameter",
          "Message": "Invalid parameter: Duplicated keys are not allowed."
        },
+       "message": "Invalid parameter: Duplicated keys are not allowed."
        "ResponseMetadata": {
          "HTTPHeaders": {},
          "HTTPStatusCode": 400
        }
      }

The message field is not present in the recorded message.
cc\ @alexrashed @thrau

Sneaked a small change in a test concerning a fake ARN which would raise the wrong Exception in AWS. Validated more tests against AWS as well.