test_object_read_unreadable

Hi,
about test `test_object_read_unreadable`(full name: `s3tests_boto3.functional.test_s3:test_object_read_unreadable`).
As I understand in this test we try to get an object which does not exist in the bucket.
The [PR](https://github.com/ceph/s3-tests/pull/156) was about the encoding utf-8 in the name of the key and the [issue](https://github.com/gaul/s3proxy/issues/201) that was opened was about head-object (and not get-object).

according to the [documentation](https://docs.aws.amazon.com/AmazonS3/latest/API/API_GetObject.html):

> If the object you request does not exist, the error Amazon S3 returns depends on whether you also have the s3:ListBucket permission. If you have the s3:ListBucket permission on the bucket, Amazon S3 will return an HTTP status code 404 ("no such key") error.

```diff
- ` eq(status, 400)`
+ ` eq(status, 404)`

- `eq(e.response['Error']['Message'], 'Couldn\'t parse the specified URI.')`
+ `eq(e.response['Error']['Message'], 'The specified key does not exist.')`
```

In case the above is true, please let me know. I can create a PR with the fix.
In case not, please specify the details so I can test in AWS s3 console (it uses boto3 also).