Improve error handling of the check-release script

- If we fail to download dist/maven files (e.g. incorrect URL, temporary
  server issue), we currently ignore it and end up with confusing errors
  about non-reproducible builds. Instead, detect the failure and error.
- Detect if the DIST_URL parameter is empty and error. This URL is
  required and leads to weird behaviors if accidentally excluded.

These changes required some refactorings, including moving the
test_files and print_results functions/colors to the top of the script
and adding a usage function