Convert PluginFailedToEncode to named fields (#11125)

# 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/shell_error.rs

@@ -745,9 +745,9 @@ pub enum ShellError {
     /// ## Resolution
     ///
     /// This is likely a bug with the plugin itself.
-    #[error("Plugin failed to encode: {0}")]
+    #[error("Plugin failed to encode: {msg}")]
     #[diagnostic(code(nu::shell::plugin_failed_to_encode))]
-    PluginFailedToEncode(String),
+    PluginFailedToEncode { msg: String },
 
     /// A message to a plugin failed to decode.
     ///