LU-13559 utils: fix lfs mirror delete error message

LU-13559 utils: fix lfs mirror delete error message

Add different error messages depending on the option used.
Add a mirror_id variable instead of reusing the id variable
to store the result of mirror_id_of(id).

Signed-off-by: Kévin Baillergeau <kevin.baillergeau.ocre@cea.fr>
Change-Id: I5fbd307c4132c22d54470f2a1407074efe8bbc0a
Reviewed-on: https://review.whamcloud.com/38609
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Dominique Martinet <dominique.martinet@cea.fr>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>