monitor/hmp-cmds: don't silently output when running 'migrate_set_downtime' fails

monitor/hmp-cmds: don't silently output when running 'migrate_set_downtime' fails

Although 'migrate_set_downtime' has been deprecated and replaced
with 'migrate_set_parameter downtime_limit', it has not been
completely eliminated, possibly due to compatibility with older
versions. I think as long as this old parameter is running, we
should report appropriate message when something goes wrong, not
be silent.

before:
(qemu) migrate_set_downtime -1
(qemu)

after:
(qemu) migrate_set_downtime -1
Error: Parameter 'downtime_limit' expects an integer in the range of 0 to 2000 seconds

Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200603080904.997083-5-maozhongyi@cmss.chinamobile.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>