Bugfix/openml connector offset

[jsmatias]

Bugfixes:

1. Issue OpenML connector doesn't recognise last asset and keeps increasing offset #266 - OpenML returns a response with status code 412, when a list is not found.

• Solved by returning a RecordError with a HttpError to the main loop. When this happens, the loop is interrupted without adding an unit to the counter, which is added to the state[offset].
• Tests added to check the state after a response with status_code=412

2. The _distributions() function was returning an empty list when all the three fields: dependencies, installation_notes, and binary_url had not None values, instead of the opposite.