Ability to only check if files are available for download

[SichongP]

I have a workflow that uses this tool to fetch fastq files of a list of user provided accessions. Sometimes a certain accession doesn't have any files associated with them. This causes download to fail and workflow to be stuck. It would be great to be able to check if a certain accession has files of specified format available, with a clear output or exit code so we can take them out and proceed with workflow. This can be done via a flag (something like --check-files-only) which outputs True/False or exit with 0/1. I can try to make a PR if this is desirable.