fix: add error messages for empty or failed exports (fixes #15)

[anjalichhikara-0907]

Problem

When running cdli export --format atf --entities artifacts, the export
silently returns an empty file with no feedback to the user. This is
confusing as users don't know if the command failed or succeeded.

Changes

• Added a check for when no entities are specified, with a clear error message
• Improved error reporting by using process.stderr.write directly for
  better visibility
• Added a summary error message when all exports fail, explaining possible
  causes
• Added a helpful note suggesting alternative formats like 'ndjson', 'csv',
  or 'ntriples' when atf export fails

Testing

Run the following to reproduce the original issue and verify the fix:

cdli export --host https://cdli.earth/ --entities artifacts --format atf

Before fix: silent empty download
After fix: clear error message with guidance on correct formats

Related

Fixes #15