Currently, when killSessionsRemote is called and the kill fails on some hosts, a vector of of these hosts (where the session was not successfully killed) is compiled (here). This vector is then logged from the router (here).

It would be an improvement to also log the specific failures from these hosts, rather than just a vector with the group of failed hosts. This was discussed being done as a part of SERVER-89630, but it would require a wider scope of changes since a number of things rely on SessionKiller's existing result type, which drops this info.