Convert FileNotFound to named fields (#11120)

# 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/env/source_env.rs

@@ -55,7 +55,9 @@ impl Command for SourceEnv {
         )? {
             PathBuf::from(&path)
         } else {
-            return Err(ShellError::FileNotFound(source_filename.span));
+            return Err(ShellError::FileNotFound {
+                span: source_filename.span,
+            });
         };
 
         if let Some(parent) = file_path.parent() {