Decodes PATH (format sniffed from its extension, or forced with
--format text|binary), reporting success or a rendered
Dextrin.Error — non-zero exit on failure, so this is meant for CI
as much as interactive use.
$ mix dextrin.validate data.dxn
$ mix dextrin.validate data.dxnb --format binary
$ mix dextrin.validate data.dxn --schema money.dxns --as Money