Insert quorum QMP events documentation alphabetically.

Also change the "ret" errno value by an optional "error" being an strerror(-ret)
in the QUORUM_REPORT_BAD qmp event.
Signed-off-by: NBenoit Canet <benoit@irqsave.net>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>