`observable_add` doesn't raise any exception and just returns `None`

[sp35]

result = self.api.observable_add(value=obs_value)

This would just return None and there is no way I can catch the exception Couldn't guess observable type as it's just logged.

[eshaan7]

Reference code: https://github.com/yeti-platform/pyeti/blob/5ec9da1bec42821869e8f4c5a8713a63c26ac111/pyeti/api.py#L684