This makes the errors point to the error location, and fixes drive_add
to report errors in the monitor instead of stderr.

While there, tweak a few error messages for consistency.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>