ImageMap should check if an external image is found/accessible

Currently, Scroll ImageMap doesn't check if an external image is present.

E.g. user can try to add an image with link such as http://apple.com/1.png. Macro doesn't display any error and allows user to add shapes and save the macro.
Upon saving the page, an error is displayed:
server Error rendering macro 'scroll-imagemap-macro' : null
cloud missing/broken image is displayed

The macro should warn user that external image is not found and grey-out Save button in such case. This would be time-saving for a user instead of waiting to display an error after the page is saved.