librbd: better error when unprotect fails on unprotected snap

This will show up on the command line and logs, making it more
clear than EINVAL.

Fixes #6851 and #4045
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1027 from jdurgin/wip-unprotect-error

librbd: better error when unprotect fails on unprotected snap

Reviewed-by: Sage Weil <sage@inktank.com>