ArcParser raises exception instead of returning error info as WarcParser does

[kngenie]

ArcParser raises exception when it encounters malformatted record:

Traceback (most recent call last):
  File "../../fixarc.py", line 30, in <module>
    offset, record, errors = a._read_record(True)
  File "/home/kenji/projects/wbm/hanzo/warctools/stream.py", line 125, in _read_record
    self.record_parser.parse(self.gz, offset=None)
  File "/home/kenji/projects/wbm/hanzo/warctools/arc.py", line 140, in parse
    headers = self.parse_header_list(line)
  File "/home/kenji/projects/wbm/hanzo/warctools/arc.py", line 185, in parse_header_list
    raise StandardError('missing headers %s %s'%(",".join(values), ",".join(self.headers)))
StandardError: missing headers http://rservicespb.ru/robots.txt,91.218.228.14,20130908181048,08Sep201318:10-1:47GMT URL,IP-address,Archive-date,Content-type,Archive-length

on the other hand, WarcParser returns tuple with error info in second element and record offset in 3rd, which is useful for locating the troubling record (and possibly automating repair process).

It'd be useful if ArcParser behaves the same way as WarcParser on error.

[tef]

Yep, it would be!

However one of the problems in the current arcparser code is that missing headers break the parser. I need to change the parser to assume the last field is always the length and hope for the best.