Explicit error messages on IIIF Import

• when an image server is unknown: give the url of the failed Image in the exception message, and ask to contact an admin (see https://gitlab.com/arkindex/requests/-/issues/332 )
• when an image is not valid: give the url of the failed Image in the exception message and ask to contact adn admin (see https://gitlab.com/arkindex/requests/-/issues/331)