Add better error handling in Baseline Mapper when an expected feature property is not found

[kflemin]

For example, if there isn't a default_template listed, the mapper will error out in a not-so-informative way. Let's try to catch the 'data' problems in the mapper and return informative error messages.

[vtnate]

@kflemin Was this resolved by #108, or is that a separate concern?