Unexpected error message when crater age not specified

[cjtu]

When running the model with a crater csv with no age specified for a particular crater, it gives the error:

"ValueError: Domain error in arguments. The `scale` parameter must be positive for all distributions, and many distributions have restrictions on shape parameters. Please see the `scipy.stats.truncnorm` documentation for details."

Perhaps we can assume a crater with no age should be assigned a uniformly random age over all model time?

Otherwise, this least this should raise a more useful error telling the user that crater ages must be specified in the csv.