LU-10921 utils: improve lfs setstripe error message

LU-10921 utils: improve lfs setstripe error message

Improve the error messages when "lfs setstripe" or "lfs setdirstripe"
is run on an existing file/directory.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I3b21fb65847822c73713e9a26d6dea978b3cab07
Reviewed-on: https://review.whamcloud.com/32442
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>