Add helpful error messages for iter_times

[rasmuse]

The iter_times function, if fed with bad input (e.g. things which cannot be multiplied or added properly), throws error messages that may be hard to understand. This situation can occure, e.g., when times are given as pandas.Timestamp but the time_unit property is not set to something compatible such as pandas.Timedelta.

It would probably be a good idea to add a simple error check in the iter_times function and add a more helpful message.