Kernel/Plan9FS: Propagate errors in Plan9FSMessage::append_data

Author: https://github.com/Freakness109 🔰
Commit: 1f1e58ed75
Pull-request: https://github.com/SerenityOS/serenity/pull/16539
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/supercomputer7 ✅

Kernel/FileSystem/Plan9FS/Message.h

@@ -135,7 +135,7 @@ public:
     Plan9FSMessage& operator<<(u32);
     Plan9FSMessage& operator<<(u64);
     Plan9FSMessage& operator<<(StringView);
-    void append_data(StringView);
+    ErrorOr<void> append_data(StringView);
 
     template<typename T>
     Plan9FSMessage& operator>>(T& t)