Print a proper error message if the input file does not exist

[shssoichiro]

Currently a missing input file will cause the encoder to panic later on. This change causes av1an to fail early with a clear error message.

[redzic]

For some reason for me this still doesn't seem to print the correct error message when the file does not exist

[shssoichiro]

flat_map was swallowing the errors from resolve_file_paths apparently and just removing them from the inputs array. Weird behavior, and honestly unexpected from the standard library.

[redzic]

I think it has to do with how filter_map is used in our read_in_dir implementation, which apparently throws out Err or None values from the iterator, and thus we end up with an empty vector. I think flat_map itself is fine

EDIT: Actually I'm not sure anymore. The read_in_dir code is really strange so I honestly think we should somehow simplify that

[redzic]

Could we just remove the resolve_file_paths function and replace the start of the parse_cli function with something like this?

  let mut input_paths = args.input;

  let mut inputs = Vec::with_capacity(input_paths.len());

  for path in input_paths.iter_mut() {
    ensure!(path.exists(), "Input path {:?} does not exist. Please ensure you typed it properly and it has not been moved.", path);

    if path.is_file() {
      inputs.push(std::mem::take(path));
    } else if path.is_dir() {
      for path in std::fs::read_dir(path.as_path())? {
        let path = path?;
        let file_type = path.file_type()?;
        if file_type.is_file() {
          inputs.push(path.path());
        }
      }
    }
  }

[redzic]

Looks good, maybe in a future PR we could remove the Box::new somehow to reduce allocations