Log more information when uploading a document containing unresolvable proxies

[gdaniel]

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Uploading a document with unresolvable proxies produces the following warning in the backend

The resource <resource_id> contains unresolvable proxies and will not be uploaded.

But no log is produced to tell the end-user or the administrator of the application what are the actual proxies that can't be resolved.

Adding this information could be helpful to let the user fix their model and upload it again.