More informative error for invalid selector values

[maxrjones]

When using this commit in #110, we got the error message Error: Missing data for chunk: -1.0.1. The issue was that an unexpected time value was passed into the selector prop. The solution was setting the units for the values passed to the selector prop to be in days since 1970-01-01 00:00:00. It would be helpful if the error message made the connection to coordinate values more obvious. Kata's suggestion was catching the invalid chunk key error and print out the valid possible coordinate values.