Nasal: improve message for non-object member access

Based on some discussion on this ticket:

https://sourceforge.net/p/flightgear/codetickets/2186/

Make this message slightly clearer.

simgear/canvas/elements/CanvasImage.cxx

@@ -947,10 +947,18 @@ SGRect<int> intersectRect(const SGRect<int>& a, const SGRect<int>& b)
     }
       
     image->setColor(color, x, y);
-    auto c = getCanvas().lock();
-    c->enableRendering(true); // force a repaint
   }
 
+    void Image::dirtyPixels()
+    {
+        osg::ref_ptr<osg::Image> image = _texture->getImage();
+        if (!image)
+            return;
+        image->dirty();
+        auto c = getCanvas().lock();
+        c->enableRendering(true); // force a repaint
+    }
+
   void Image::allocateImage()
   {
     osg::Image* image = new osg::Image;

simgear/canvas/elements/CanvasImage.hxx

@@ -115,6 +115,11 @@ namespace canvas
       
       void setPixel(int x, int y, const osg::Vec4& color);
 
+      /**
+        * mark the image pixels as modified, so the canvas is re-painted
+       */
+      void dirtyPixels();
+      
       osg::ref_ptr<osg::Image> getImage() const;
       
     //  void setRow(int row, int offset, )

simgear/nasal/code.c

@@ -503,7 +503,7 @@ static void setMember(naContext ctx, naRef obj, naRef fld, naRef value)
         return;
     }
     
-    if(!IS_HASH(obj)) ERR(ctx, "non-objects have no members");
+    if(!IS_HASH(obj)) naRuntimeError(ctx, "non-object does not have member: %s", naStr_data(fld));
     naHash_set(obj, fld, value);
     ctx->opTop -= 2;
 }