job.download_files should provide feedback when no files are available to download

[asjohnston-asf]

Is your feature request related to a problem? Please describe.
Currently in this situation download_files() prints a progress bar without data and exists successfully. It's not immediately intuitive from the output that there were no files available to download.

> file_list = jobs.download_files()
0/0 [00:00<?, ?it/s]

Describe the solution you'd like
Logging a warning message could provide more specific feedback in this situation.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.