As an afterthought, it's not great that this introduces a different message for the same error on multiple platforms (ie, Windows says 'could not find file')

I can confirm that this does improve the error message on my machine as well

error: cannot open directory: build
  in file/open [src/core/io.c] on line 153
  in dofile [boot.janet] on line 3002, column 12
  in cli-main [boot.janet] on line 4645, column 17

While I like the error message, something bothers me about making an extra system call every time we open a file. That said, python does the same thing, and there is always os/open for less abstracted IO.

Being a system call didn't occur to me, but I agree, though looking at the field it seems everyone is fine doing this, which is crazy to me...open/fopen give multiple options to ensure a target is definitely a directory; I'm very curious what the use case is to 'read' a directory in binary...everything I've read suggests nobody actually does that.

Alternatively, as Jakob originally suggested just save the file name for use later if an error pops up. I assume nobody can open enough files to make the memory usage matter?