Add exception to setImageMetadataForHandle

This commit adds a check for imageMetadata for handle in case, there
already exists an entry in the zoomLeveltoImageHandle map for a zoom
level and it must throw an exception in that case.

contributes to #62 and #127

Author: amartya4256

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/Image.java

@@ -1788,9 +1788,11 @@ else if (i.alphaData != null) {
 }
 
 private void setImageMetadataForHandle(ImageHandle imageMetadata, Integer zoom) {
-	if (zoom != null && !zoomLevelToImageHandle.containsKey(zoom)) {
-		zoomLevelToImageHandle.put(zoom, imageMetadata);
+	if (zoom == null) return;
+	if(zoomLevelToImageHandle.containsKey(zoom)) {
+		SWT.error(SWT.ERROR_ITEM_NOT_ADDED);
 	}
+	zoomLevelToImageHandle.put(zoom, imageMetadata);
 }
 
 static long [] init(Device device, Image image, ImageData source, ImageData mask, Integer zoom) {