lightnvm: print error when target is not found

If userspace requests target to be removed, nvm_remove_tgt() will
iterate the nvm_devices to find out the given target, but if not
found, then it should print out an error.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Updated output string and patch description.
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>