Convert FileNotFoundCustom to named fields (#11123)

# Description

Part of #10700

# User-Facing Changes

None

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu-command/src/filesystem/cp.rs

@@ -563,9 +563,15 @@ fn convert_io_error(error: std::io::Error, src: PathBuf, dst: PathBuf, span: Spa
     let shell_error = match error.kind() {
         ErrorKind::NotFound => {
             if std::path::Path::new(&dst).exists() {
-                ShellError::FileNotFoundCustom(message_src, span)
+                ShellError::FileNotFoundCustom {
+                    msg: message_src,
+                    span,
+                }
             } else {
-                ShellError::FileNotFoundCustom(message_dst, span)
+                ShellError::FileNotFoundCustom {
+                    msg: message_dst,
+                    span,
+                }
             }
         }
         ErrorKind::PermissionDenied => match std::fs::metadata(&dst) {