Update downloader.sh

[fabriziosalmi]

Helo my Lord.... I'm trying to contribute to this gem, then I wanna check if PR are welcome just by proposing a simple update for a single downloader, if appreciated I can cover all downloaders quickly :)

List of improvements:

• Dependency check: Ensures curl, dig, jq, and mktemp are installed before proceeding.
• Error handling in downloads: Added retries and error messages for failed downloads.
• Background download: Parallelized downloads for faster execution using & and wait.
• Error handling in dig calls: Added fallback || true to prevent script failure if DNS resolution fails.
• Recursive SPF record resolution: Improved robustness with additional error handling.
• Temp directory with trap: Ensured that the temporary directory is always cleaned up upon script exit.
• Improved sorting: Used sort -u for efficient sorting and deduplication in one step.
• Output directory creation: Ensured the output directory is created before saving the results.
• File size check: Added validation to check if the output files are empty or failed to generate.
• Logging: Enhanced logging for error messages and successful file creations.