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-protocol/src/engine/engine_state.rs

@@ -454,11 +454,16 @@ impl EngineState {
         // Updating the signatures plugin file with the added signatures
         self.plugin_signatures
             .as_ref()
-            .ok_or_else(|| ShellError::PluginFailedToLoad("Plugin file not found".into()))
+            .ok_or_else(|| ShellError::PluginFailedToLoad {
+                msg: "Plugin file not found".into(),
+            })
             .and_then(|plugin_path| {
                 // Always create the file, which will erase previous signatures
-                std::fs::File::create(plugin_path.as_path())
-                    .map_err(|err| ShellError::PluginFailedToLoad(err.to_string()))
+                std::fs::File::create(plugin_path.as_path()).map_err(|err| {
+                    ShellError::PluginFailedToLoad {
+                        msg: err.to_string(),
+                    }
+                })
             })
             .and_then(|mut plugin_file| {
                 // Plugin definitions with parsed signature
@@ -510,11 +515,15 @@ impl EngineState {
                             // information when nushell starts
                             format!("register {file_name} {shell_str} {signature}\n\n")
                         })
-                        .map_err(|err| ShellError::PluginFailedToLoad(err.to_string()))
+                        .map_err(|err| ShellError::PluginFailedToLoad {
+                            msg: err.to_string(),
+                        })
                         .and_then(|line| {
-                            plugin_file
-                                .write_all(line.as_bytes())
-                                .map_err(|err| ShellError::PluginFailedToLoad(err.to_string()))
+                            plugin_file.write_all(line.as_bytes()).map_err(|err| {
+                                ShellError::PluginFailedToLoad {
+                                    msg: err.to_string(),
+                                }
+                            })
                         })
                         .and_then(|_| {
                             plugin_file.flush().map_err(|err| {

crates/nu-protocol/src/shell_error.rs

@@ -736,9 +736,9 @@ pub enum ShellError {
     /// ## Resolution
     ///
     /// This is a fairly generic error. Refer to the specific error message for further details.
-    #[error("Plugin failed to load: {0}")]
+    #[error("Plugin failed to load: {msg}")]
     #[diagnostic(code(nu::shell::plugin_failed_to_load))]
-    PluginFailedToLoad(String),
+    PluginFailedToLoad { msg: String },
 
     /// A message from a plugin failed to encode.
     ///