Improve error handling when icons lack CORS headers

[simonbrowndotje]

Description

The lack of CORS headers on images/logos/icons will cause PNG diagram exports to fail. This is mentioned in the documentation (branding, icons, etc), but errors are hidden in the browser's JavaScript console. It would be good to have these errors more visible.

Priority

High (we have a paid support contract - please e-mail us with your support contract details)

More information

I've looked at this before, and the errors were not easily catchable given their nature and where they originate. If it's still not possible to catch the errors, perhaps some workspace validation could be added instead.

[simonbrowndotje]

The fix worked via localhost URLs, but not when deployed to the cloud service unfortunately.