Throw error if current user is not found

[christoph-blessing]

Right now the get_current_user method in the Fabrikant class returns None if it doesn't find an entry in the Fabrikant table corresponding to the current Datajoint username. This behavior of returning None can lead to non-helpful error messages in certain situations. For example trying to insert a dataset into the Dataset table without a matching entry being present in the Fabrikant table raises this error:

MissingAttributeError: Field 'dataset_fabrikant' doesn't have a default value

I propose to raise a more descriptive error in the get_current_user method if there is no matching entry found.