Avoid crashing upon invalid timestamps

Traceback (most recent call last):
  File "rfc_stats.py", line 547, in <module>
    for i, vote in enumerate(vote_page.get_votes()):
  File "rfc_stats.py", line 303, in get_votes
    yield Vote.from_line(self, line, section_label)
  File "rfc_stats.py", line 89, in from_line
    vote.datetime = cls.parse_datetime(line)
  File "rfc_stats.py", line 125, in parse_datetime
    return datetime.strptime(m.group(0), config.date_format)
  File "/usr/lib64/python2.7/_strptime.py", line 325, in _strptime
    (data_string, format))
ValueError: time data '24:14, 2 March 2014 (UTC)' does not match format '%H:%M, %d %B %Y (UTC)'