check-news: fix behavior on invalid date line
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 <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

mbakke/ganeti,apyrgio/snf-ganeti,ganeti-github-testing/ganeti-test-1,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,grnet/snf-ganeti,onponomarev/ganeti,apyrgio/ganeti,ganeti/ganeti,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti,andir/ganeti,dimara/ganeti,ganeti/ganeti,bitemyapp/ganeti,andir/ganeti,leshchevds/ganeti,andir/ganeti,mbakke/ganeti,leshchevds/ganeti,yiannist/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,grnet/snf-ganeti,apyrgio/ganeti,leshchevds/ganeti,yiannist/ganeti,dimara/ganeti