[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>

Expected behavior

• a 404 or equivalent message
• Tests verifying the fix are added

Additional context

Estimation of size: small

Estimation of priority: low