yaml: miscellaneous improvements

User-facing:

* Reports errors for track refs referencing rereleases instead of
  original tracks. (Resolves #247)
* Reports YAML field names instead of property names.
* Reports invalid group refs under sourceGroupByRef.

Code-facing:

* Cleanup to make it easier to add new "custom" reference error
  reporting functions; now implemented as custom findFn, which
  will automatically adapt to array or non-array property values
  just like normal findFn.
* Reports invalid property keys, i.e. value === undefined instead
  of value === null.

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