Revert "qemu: monitor: do not report error on shutdown"

Revert "qemu: monitor: do not report error on shutdown"

https://bugzilla.redhat.com/show_bug.cgi?id=1536461

This reverts commit aeda1b8c56dc58b0a413acc61bbea938b40499e1.

Problem is that we need mon->lastError to be set because it's
used all over the place. Also, there's nothing wrong with
reporting error if one occurred. I mean, if there's a thread
executing an API and which currently is talking on monitor it
definitely wants the error reported.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>