Confused active attributes returned if not running detect_errors before generate domain

[zaqthss]

If hc.setup_domain() runs without hc.detect_errors(detectors) in advance, the active_attributes as well as the dk cells for domain generation will be read from the postgres database (AuxTables.dk_cells.name) directly, which may cause the error of no field of name XXX.

We should explicitly show that when using holoclean for a dataset, we must either run the detect errors or load the dk cells from a specific relation

[minafarid]

An alternative is to try to read active_attributes and fall back to all attributes in the dataset if no dk_cells table is available.