datalad special remote's reporting on error causes is suboptimal

[bpoldrack]

Just observed a user getting Failed to download from any of 2 locations [None].
This gives no idea what the None would be about. It's actual purpose is to record the __cause__ of the original exception raised by the download call within the special remote's transfer_retrieve.

[yarikoptic-gitmate]

Issue fixed in 0.18.3