Remove println error handling for graph loaders and return results instead

[ljeub-pometry]

We have a bunch of places where we handle errors by matching on a result and turning the error into a println. We should fix that and handle things properly.

This seems to be the case for most of the graph loaders but there are likely also other places.

[ljeub-pometry]

These println! are now error! logging outputs, but that is barely an improvement.

[miratepuffin]

@arienandalibi if this doesn't refer to the df loaders pelase close