Ensure that any missing or incorrectly specified key in core.yaml results in an appropriate error message

[davidkel]

Looking at some of the code, I have concerns that there isn't enough error checking on missing entries or incorrectly specified entries in core.yaml that could result in cryptic errors or unhelpful panic messages (eg index out of range).

This issue is about testing and adding checks to ensure that this is the case

(issues where optional keys are mandatory should also be addressed as well)

kind of a followon to #406