Closing non-existing workspace

When running bst workspace close --remove-dir core/gnome-keyring.bst, the user is asked if they really want to remove the directory, because it can imply loss of work.

After answering "Yes I wanna delete that...", BuildStream errors out with:

ERROR: Workspace 'core/gnome-keyring.bst - 0' is currently not defined

This is backwards, the error message should come before ever prompting the user.