refactor: improve error handling

src/ro.h

@@ -41,9 +41,9 @@ namespace ro
             return std::string(l.what()) == std::string(r.what());
         }
 
-        inline Error operator+(const Error &other)
+        friend inline Error operator+(const Error & l, const Error &r)
         {
-            return Error(std::string(this->what()) + "\n" + other.what());
+            return Error(std::string(l.what()) + "\n" + r.what());
         }
     };
 
@@ -346,7 +346,7 @@ namespace ro
             throw m_result;
         }
 
-        inline const T &expect(E e) const
+        inline const T &expect(const E& e) const
         {
             if (is_ok())
             {