qdev-monitor: Convert qbus_find() to Error

As usual, the conversion breaks printing explanatory messages after
the error: actual printing of the error gets delayed, so the
explanations precede rather than follow it.

Pity.  Disable them for now.  See also commit 7216ae3d.

While there, eliminate QERR_BUS_NOT_FOUND, and clean up unusual
spelling in the error message.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>