fix bug 1065071 - ignore unrecognizable JSON files

Actually, this KeyError is ambiguous. It could happen due to `kvpairs['moz_source_repo']`, `kvpairs['moz_update_channel']` or `kvpairs['buildid']`.
How about doing it like this:
```python
try:
    kvpairs = json.loads(content)
    if 'moz_source_repo' not in kvpairs:
        # bug 1065071 - JSON structs that lack the `moz_source_repo` is
        # something we currently can't support
        return
    kvpairs['repository'] = kvpairs['moz_source_repo'].split('/', -1)[-1]
    ...
```