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

Author: shssoichiro

av1an-cli/src/lib.rs

@@ -541,6 +541,8 @@ pub fn parse_cli(args: CliOpts) -> anyhow::Result<Vec<EncodeArgs>> {
   let mut valid_args: Vec<EncodeArgs> = Vec::with_capacity(inputs.len());
 
   for input in inputs {
+    ensure!(input.exists(), "Input file {:?} does not exist. Please ensure you typed it properly and it has not been moved.", input);
+
     let temp = if let Some(path) = args.temp.as_ref() {
       path.to_str().unwrap().to_owned()
     } else {