s3 website ignore --error-document argument

When both `--index-document` and `--error-document` options are supplied,
`--error-document` argument was ignored.

How to reproduce

```
$ aws s3 mb s3://b0e1.example.com
make_bucket: s3://b0e1.example.com/
$ aws s3 website s3://b0e1.example.com --index-document  test.html --error-document error.html
$ aws s3api get-bucket-website --bucket b0e1.example.com
{
    "RedirectAllRequestsTo": {},
    "IndexDocument": {
        "Suffix": "test.html"
    },
    "ErrorDocument": {}, # <- ignored!
    "RoutingRules": []
}
```