log access problems to the /entity_share endpoint

Problem/Motivation

It is REALLY HARD to debug access problems with Entity Share. The pull form can show that there is no access to channels, and there's no way to find out why.

For instance, one problem I've hit several times is BasicAuth hitting the flood limit, and denying access to the client site's IP. This is hard to figure out, in particular because it can't be reproduced on a different environment.

Steps to reproduce

Proposed resolution

Can we listen on the server for 401s for the /entity_share endpoint, and log them?

Remaining tasks

User interface changes

API changes

Data model changes