[Feature Request]: Improve error logging in download-server

[parkan]

What is the problem you're trying to solve?

the download-server process does not appear to emit any errors when downloads fail

Describe the workaround you currently have

we can manually run singularity download for the same piece and capture the error

Describe the feature you'd like

the download-server should report errors just as download does

Additional context

No response

[parkan]

furthermore, download-server currently sends 500 if the metadata service is not available, which is both not compliant with https://www.rfc-editor.org/rfc/rfc9110.html semantics and unhelpful for debugging (for example it does not help us distinguish remote server 500 errors from failures to connect to remote server). the error handlers in https://github.com/data-preservation-programs/singularity/blob/main/service/downloadserver/downloadserver.go#L112C26-L112C40 should most likely raise 400.