Log failed summary builds

A build of a summary node that fails logs one line with the node and the
cause, and tells that the build is tried again at the next message. A
build that a stop of the daemon ends logs nothing. The error of the
compactor holds no node, because the line names it once.

The tests give each memory a logger, and a shared helper gives the ones
that the tests do not read.

Issue: e4acfcb