tape: improve export_media error message for not found tape

tape: improve export_media error message for not found tape

'export_media' can handle if the tape is in either a normal slot of the
library, or in the drive assigned to the current pool writer.
(because we need to lock the drive)

if it is, for some reason, in a different drive, the error message
 'media is not online'
could be slightly confusing for a user, since it would appear in the drive list

add the 'or a differen drive' to make it clearer

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>