stop assuming paths are files in error messages
stop assuming paths are files in error messages

Changed error messages to talk about "files and directories" or "paths" instead of just "files".
If a user misspells a directory, a message saying "File does not exist" is confusing and suggests that elm-format cannot handle directories (it can).
avh4/elm-format,avh4/elm-format,avh4/elm-format,avh4/elm-format