Added try/except for "Errata" KeyError

When I tried using ocperf, I was getting this error:
```
Traceback (most recent call last):
  File "./ocperf.py", line 561, in <module>
    emap = find_emap()
  File "./ocperf.py", line 403, in find_emap
    emap = json_with_offcore(el, True)
  File "./ocperf.py", line 370, in json_with_offcore
    emap = EmapNativeJSON(event_download.eventlist_name(el, "core"))
  File "./ocperf.py", line 333, in __init__
    return self.read_table(data, mapping)
  File "./ocperf.py", line 241, in read_table
    if get('errata') != "null":
  File "./ocperf.py", line 185, in <lambda>
    get = lambda (x): row[m[x]]
KeyError: u'Errata'
```

I wasn't sure if there was ever a case when the Errata field existed with the string "null" so I decided to try/except around the whole call. For reference, my system is using the GenuineIntel-6-2C-core.json file.

Thanks!