Simplify link checker

... and debug a bit.

check-links.py

@@ -8,12 +8,12 @@
 
 refs = [ref for ref in pdf.get_references() if ref.reftype == 'url']
 
-status_codes = list(map(lambda ref: pdfx.downloader.get_status_code(ref.ref), refs))
+status_codes = [pdfx.downloader.get_status_code(ref.ref) for ref in refs]
 
-broken_links = [refs[idx].ref for idx in range(len(refs)) if status_codes[idx] != 200]
+broken_links = [(ref.ref, code) for ref, code in zip(refs, status_codes) if code != 200]
 
 # it seems that Twitter does not respond well to the link checker and throws a 400
-if all(['twitter.com' in url for url in broken_links]):
+if all(['twitter.com' in url for url, _ in broken_links]):
     sys.exit(0)
 else:
     print('Broken links:', broken_links)