ListRecords ignores addresses with non-OAI content

```python
while True:
  try:
    sickle = Sickle('https://furkankalkan.com')
    records = sickle.ListRecords(metadataPrefix='oai_dc', ignore_deleted=True)
    records.next()
  except StopIteration:
    break
  except Exception as e:
    raise e
```

Content of my site is HTML, but it seems sickle doesn't throw an error when encountering non-OAI-PMH content
