[download] Server propagates the s3-backend feedback message to the user if a file/dataset does not exist

[aaperis]

Describe the bug
Trying to fetch a non-existent file with sda-download returns an s3-backend-propagated message to the user.

Steps to reproduce
the following:

curl -s --location 'https://staging-download.gdi.nbis.se/s3-encrypted/someDatasetID/someFileID' --header "Authorization
: Bearer $token"

returns

<?xml version="1.0" encoding="UTF-8"?>
<ListAllMyBucketsResult><Buckets></Buckets><Owner></Owner></ListAllMyBucketsResult>

with code 200 whereas an s3 backend would give:

HTTP/1.1 404 Not Found
< content-length: 233
< x-amz-request-id: tx0000074b017e3d72253d7-006735bc6d-6a91224-default
< accept-ranges: bytes
< content-type: application/xml
< date: Thu, 14 Nov 2024 09:01:33 GMT
< 
* Connection #0 to host storage.sto3.safedc.net left intact
<?xml version="1.0" encoding="UTF-8"?><Error><Code>NoSuchBucket</Code><BucketName>gdi-staging-publi</BucketName><RequestId>tx0000074b017e3d72253d7-006735bc6d-6a91224-default</RequestId><HostId>6a91224-default-default</HostId></Error>

Expected behavior

• a 404 status returned, mimicking s3 backend behavior e.g. see above
• Tests verifying the fix are added

Additional context

Estimation of size: small

Estimation of priority: low