Log failed turns and the ends of subagents

A turn that ends with an error logs one line with the error that the
clients get, such as a needed login or HTTP 429, so that it is not lost
when no client is connected. An interrupt of the user or a stop of the
daemon logs nothing.

The end of each subagent run logs one line with the name of the
subagent: done, failed with the cause, or stopped by the user. A cut
subagent that cannot go on after a restart logs its cause too.

Issue: e4acfcb