ArcParser raises exception instead of returning error info as WarcParser does

ArcParser raises exception when it encounters malformatted record:

```
Traceback (most recent call last):
File "../../fixarc.py", line 30, in
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.