warn user if no sources or collections specified

[rahulbot]

Another tiny change - add a warning if the user queries with no sources or collections. Maybe they'll see it, maybe they won't, but it's a first step in advising better usage when they use the API client. Includes unit test.

Other ideas we've discussed:

• throwing an error in api client to not allow this kind of usage
• adding a check on server to reject no-source/no-collection queries unless user is admin