More informative exception message for only

More informative exception message for only: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). For example (as noted by my colleague @berquist ), compare get only…

The task

For example (as noted by my colleague @berquist ), compare [`get_only`](https://github.com/isi-vista/vistautils/blob/7eb87c06d3dc31d8c29f43d932ef6b834500c3f2/vistautils/collection_utils.py#L8) to…

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.