Display CORS issue

Refs https://gitlab.com/arkindex/requests/-/issues/531

I'm not sure that's possible at all (and would not be surprised if not).

STR:

• go on import files
• input https://nubis.univ-paris1.fr/files/original/BIS_05_00025_P001_0001.jpg in the url
• click on validate
• 💥 network error due to CORS

So that's fine, we won't ever get CORS approval on many servers, but if we could display a message stating it's the fault of the server and not ours, that would be great.

Sample message: The external image server does not support remote fetch (CORS)