Add more checks for metadata contents

[karawoo]

Check values that correspond to annotations and provide feedback about invalid values (and where to find the correct ones)

[larssono]

Do you already have decided on how the location and values of correct ones will be referenced? I.e. how will these be defined outside of the code.

[karawoo]

I was planning to use what's defined in https://github.com/Sage-Bionetworks/synapseAnnotations

This is already what we use for validating the file annotations, so it's just a matter of applying the same checks to the contents of the files themselves. However, it's possible we'd want to add additional requirements beyond what's used for annotations. Where/how to store that hasn't yet been discussed or decided.