btrfs-progs: fi show: print error message if no valid Btrfs is specified

btrfs-progs: fi show: print error message if no valid Btrfs is specified

* Before this patch

 ===============================
 # ./btrfs fi show foo      # "foo" doesn't mean any valid Btrfs
 #                          # no error message
 # echo $?
 1
 ===============================

* After this patch

 ===============================
 # ./btrfs fi show foo
 ERROR: foo is not a valid Btrfs
 #
 # echo $?
 1
 ===============================

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>