Added exception check in __contains__ in activityLog class

This fix prevents unexpected exceptions in `in` checks:
```python
info = instance.get_activitylog(...)
"some string" in info  # throws ApiNotFoundError if "some string" is not in info
```

It is probably better to fix `find()` method but its current behavior may be used somewhere, while I severely doubt that the same behavior of `in` construct is relied onto anywhere.
Ok, I'll update `find()` method then.