NestDefaults.Load caught only JsonException and IOException, so a
defaults.json that File.Exists can see but the process cannot read
threw UnauthorizedAccessException out of New. Report such files as
Invalid and return the fallback values, like a corrupt file.
The regression mode-000s the file on Unix (skipped on Windows and when
running as root); it failed with UnauthorizedAccessException before
this change.
aj
39a15aee14