Remove DMatrix option bug in UI

Do not change value of option3 if user has selected a Data Matrix symbol size.
If a warning is generated when saving, make this visible to user.
Fixes #85 reported by Harald Oehlmann

backend_qt/qzint.h

@@ -70,6 +70,8 @@ public:
 
     int securityLevel() const;
 	void setSecurityLevel(int securityLevel);
+        
+        int getError();
 
     float scale() const;
 	void setScale(float scale);