VideoCommon: Reword the unknown opcode error message

When faced with this error, users often don't try disabling dual core,
even though the error message suggests it. Perhaps the message is just
too long and lists too many things?

To try to improve the situation, I'm rewording the message and making it
say different things depending on what settings you are using.

Source/Core/VideoCommon/OpcodeDecoding.cpp

@@ -219,8 +219,8 @@ public:
     }
     else
     {
-      Core::System::GetInstance().GetCommandProcessor().HandleUnknownOpcode(opcode, data,
-                                                                            is_preprocess);
+      auto& system = Core::System::GetInstance();
+      system.GetCommandProcessor().HandleUnknownOpcode(system, opcode, data, is_preprocess);
       m_cycles += 1;
     }
   }