check-news: fix behavior on invalid date line

Before:
$ ./autotools/check-news < ./NEWS
Traceback (most recent call last):
  File "./autotools/check-news", line 150, in <module>
    main()
  File "./autotools/check-news", line 120, in main
    parsed_ts = time.mktime(time.strptime(m.group("date"), "%d %b %Y"))
AttributeError: 'NoneType' object has no attribute 'group'

After:
$ ./autotools/check-news < ./NEWS
Line 8: Invalid release line

This happened because the error function just updates an error log,
rather than bailing out immediately.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>