Give an exact error message when the file is not found

[dmikushin]

When the folder does not exist, facad's error reporting is mesleading:

►ls test
Error: Unexpected argument 'test'
Usage: ls [OPTION] [DIRECTORY]
Try 'ls --help' for more information.

It would be very useful to have a more exact error message about invalid file or directory:

►ls.coreutils test
ls.coreutils: cannot access 'test': No such file or directory

[yellow-footed-honeyguide]

Thank you, @dmikushin ! I fixed it.