Convert PluginFailedToLoad to named fields (#11124)

# 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-plugin/src/protocol/mod.rs

@@ -83,7 +83,7 @@ impl From<ShellError> for LabeledError {
                 msg: format!("did you mean '{suggestion}'?"),
                 span: Some(span),
             },
-            ShellError::PluginFailedToLoad(msg) => LabeledError {
+            ShellError::PluginFailedToLoad { msg } => LabeledError {
                 label: "Plugin failed to load".into(),
                 msg,
                 span: None,

crates/nu-plugin/src/protocol/plugin_custom_value.rs

@@ -68,9 +68,9 @@ impl CustomValue for PluginCustomValue {
             let stdout_reader = match &mut child.stdout {
                 Some(out) => out,
                 None => {
-                    return Err(ShellError::PluginFailedToLoad(
-                        "Plugin missing stdout reader".into(),
-                    ))
+                    return Err(ShellError::PluginFailedToLoad {
+                        msg: "Plugin missing stdout reader".into(),
+                    })
                 }
             };
             get_plugin_encoding(stdout_reader)?