Improve AUCell error messages when loading an empty regulons file

[cflerin]

Currently, when the regulons file is empty:

,,Enrichment,Enrichment,Enrichment,Enrichment,Enrichment,Enrichment,Enrichment,Enrichment
,,AUC,NES,MotifSimilarityQvalue,OrthologousIdentity,Annotation,Context,TargetGenes,RankAtMax
TF,MotifID,,,,,,,,

AUCell (CLI) fails with:

2020-02-10 14:04:37,880 - pyscenic.cli.pyscenic - INFO - Loading expression matrix.

2020-02-10 14:04:37,897 - pyscenic.cli.pyscenic - INFO - Loading gene signatures.

2020-02-10 14:04:37,907 - pyscenic.cli.pyscenic - ERROR - Cannot set a frame with no defined index and a value that cannot be converted to a Series
Create regulons from a dataframe of enriched features.
Additional columns saved: []

It would be helpful to have a more descriptive error