TypeError (str/bytes) in warc.py error path

In production at IA, probably caused by petabox downtime or network error, I got a the following exception and stack trace:

```
TypeError: sequence item 0: expected str instance, bytes found
File "extraction_ungrobided.py", line 272, in
MRExtractUnGrobided.run()
File "mrjob/job.py", line 424, in run
mr_job.execute()
File "mrjob/job.py", line 433, in execute
self.run_mapper(self.options.step_num)
File "mrjob/job.py", line 517, in run_mapper
for out_key, out_value in mapper(key, value) or ():
File "extraction_ungrobided.py", line 228, in mapper
info, status = self.extract(info)
File "extraction_ungrobided.py", line 143, in extract
info['file:cdx']['c_size'])
File "extraction_ungrobided.py", line 126, in fetch_warc_content
gwb_record = rstore.load_resource(warc_uri, offset, c_size)
File "wayback/resourcestore.py", line 65, in load_resource
return create_resource(loader.load_block(bstart, blen))
File "wayback/resource.py", line 583, in create_resource
record, errors, offset = parser.parse(rs, 0, line)
File "hanzo/warctools/warc.py", line 223, in parse
% (",".join(self.KNOWN_VERSIONS)),
```

`self.KNOWN_VERSIONS` is defined as bytes at https://github.com/internetarchive/warctools/blob/master/hanzo/warctools/warc.py#L177, but is being joined with a string.

One fix, though i'm not sure it would work in Python 2.7, would be:

```(",".join([s.decode('utf-8') for s in self.KNOWN_VERSIONS])```

There's probably a more idiomatic way, but I can submit a patch for that.

While we're at it, might want to make it a join on ", ", not ","?

Start in hanzo/warctools/warc.py at parse around line 223 and inspect KNOWN_VERSIONS around line 177. Check the reported bytes/string mismatch and Python 2.7 compatibility, then look for a relevant parser test or reproduction path. Done means the WARC error path no longer raises this TypeError and still reports the known versions.

Written by the indexing model from the issue text.