copy_files: should Dune error out if no files are found?

[jchavarri]

Right now, Dune is lenient in the way copy_files work. If I do:

(copy_files foo/bar.txt)

But if the file foo/bar.txt doesn't exist, Dune will proceed just fine, as far as I can tell.

I wonder if it should error out if no files to copy are found given a glob? So that users can also get some help to maintain their dune files clean and avoid wasteful work for Dune (parsing, resolving globs, looking for inexistent files...)

[rgrinberg]

Raising an error in newer versions of dune seems fine. We could always introduce a mode to disable this check if somebody will need it.