[download] Erroneous urls should return url not found

[aaperis]

How to reproduce:

./sda-cli list --config s3cmd-bp.conf -url https://whatever.bp.nbis.se -datasets
The provided access token expires in 719h33m2s
Error: failed to get datasets, reason: failed to get response, reason: Get "https://whatever.bp.nbis.se/metadata/datasets": tls: failed to verify certificate: x509: certificate is valid for ingress.local, not whatever.bp.nbis.se

and

./sda-cli list --config s3cmd-bp.conf -url https://whatever.nbis.se -datasets
The provided access token expires in 719h32m53s
Error: failed to get datasets, reason: failed to get response, reason: Get "https://whatever.nbis.se/metadata/datasets": dial tcp: lookup whatever.nbis.se: no such host

Expected behavior:

• All erroneous urls should not propagate internal errors to the user but return url not found instead.