Incorrect handling of default inheritance

xref: https://github.com/galaxyproject/tpv-shared-database/pull/62

From @mvdbeek
> guess that puts a couple of items on my whishlist:
> * raise a well-defined exception indicating a temporary failure in calculating the job destination
> * make it so that we don’t fail the job when we encounter that exception … just retry later if possible
> * produce some kind of type interface for a given variable and validate all rules / code blocks against them