rgw/rest: wrap iam/sns/sts Error responses with ErrorResponse

rgw/rest: wrap iam/sns/sts Error responses with ErrorResponse

all iam/sns/sts requests wrap the s3 <Error> xml response in another
<ErrorResponse xmlns="https://iam.amazonaws.com/doc/2010-05-08/">

without this, boto3 fails to fully parse error responses, leading to
generic Unknown ClientError exceptions of the form:
    botocore.exceptions.ClientError: An error occurred (Unknown) when calling the PutUserPolicy operation: Unknown

with the ErrorResponse part, boto3 throws more specific exceptions that
include the error Code and Message:
    botocore.errorfactory.NoSuchEntityException: An error occurred (NoSuchEntity) when calling the PutUserPolicy operation: No such UserName in the account

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 195ecb3732e0d7f78dc74cc9b7f2ac3d7ed919a0)