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>

autotools/check-news

@@ -113,6 +113,8 @@ def main():
       m = RELEASED_RE.match(line)
       if not m:
         Error("Line %s: Invalid release line" % fileinput.filelineno())
+        expect_date = False
+        continue
 
       # Including the weekday in the date string does not work as time.strptime
       # would return an inconsistent result if the weekday is incorrect.