Specify which file not found in error (#11868)

# Description
Currently, `ShellError::FileNotFound` shows the span where the error
occurred but doesn't say which file wasn't found. This PR makes it so
the help includes that (like the `DirectoryNotFound` error).

# User-Facing Changes
No breaking changes, it's just that when a file can't be found, the help
will say which file couldn't be found:


![image](https://github.com/nushell/nushell/assets/45539777/e52f1e65-55c1-4cd2-8108-a4ccc334a66f)

crates/nu-command/src/system/nu_check.rs

@@ -120,6 +120,7 @@ impl Command for NuCheck {
                                 path
                             } else {
                                 return Err(ShellError::FileNotFound {
+                                    file: path_str.item,
                                     span: path_str.span,
                                 });
                             }