InputConfigDiag: Add a simple error status label

This tells you what you did wrong at a high level if you messed up.

Source/Core/DolphinWX/Src/InputConfigDiag.h

@@ -112,6 +112,7 @@ public:
 private:
 	GamepadPage* const		m_parent;
 	wxStaticText*		m_bound_label;
+	wxStaticText*		m_error_label;
 	DeviceQualifier	m_devq;
 	bool GetExpressionForSelectedControl(wxString &expr);
 };