Revert "Improve error handling of dataset names in region_series"

[dramanica]

Reverts #56

[dramanica]

@japilo Ouch, the dev vs master ended up being an issue. I am afraid I have to revert your changes, as they seem to create a number of problems with dev. We'll have to figure out where those issues come from. Your checks seem to trigger a number of datasets that were working before.

[japilo]

I'm kind of surprised! The datasets I wrote these checks for were datasets that were not working with the location_slice* functions. They were just there to provide informative error messages for things that were already problematic.

[dramanica]

Sorry, misunderstanding, the problem is that datasets that were used in dev, which work fine at the moment, trigger the checks that you created for misshaped names. I am guessing that it might be an issue with empty varnames triggering the checks, or something like that.