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.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information