DBS session.save should not raise a concurrent exception in case of missing proxy

On DBS session.save proxies are updated, if the list of proxies contains an inexistant proxy a concurrent exception is raised.

On doc 06b8ac3b-8a7a-4c0c-bfca-0358e37b9599, Proxy 691e5f42-4de1-b06f-ae288e228497 concurrently deleted.

This can be the case on concurrent deletion of proxies, but if the invalid list of proxy is saved, there is no way to fix the document.

Instead of raising an exception an error log should be enough.