* Additional GAGS configuration json verification (#59524)
This standardizes how values are read from the json for greyscale layers so that error handling can check for some additional things:
No extra keys in the json that are unknown
No missing keys that a layer needs to work
Values are the expected type for that key
A variety of error messages have been added for various ways the json can be malformed and should hopefully provide good feedback for anyone working with greyscale configurations.
* Additional GAGS configuration json verification
Co-authored-by: Emmett Gaines <ninjanomnom@gmail.com>
