[BUG] setting timestampFormat/dateFormat for JsonToStructs appears to fall back too often and also not often enough

[revans2]

Describe the bug
I am not 100% sure what is happening here as there is code in place for this, and there appears to be some tests in ScanJson for it. But the tests and the checks don't look like they are compatible with each other and every time I try to set the timestampFormat, for from_json it fallback back to the CPU. I don't know why. Even if I set it to the one hard coded value that should work.

But for dateFormat I am able to set it to something like MMM yyyy which is a format that we don't support, and it does not fall back to the CPU. Instead we get an error about how when we try to convert it to the format that CUDF wants it is not supported.