unable to delete branch with forgotten bisect

Hi.

Trying to delete the branch, I
was getting this error:

error: cannot delete branch '<branch_name>' used by worktree at 
'<wtree_path>'

It wasn't used by that work-tree
directly, so I was very confused.
It appears this branch had the
forgotten bisect.

It would be very good if git can
write a more prominent message
for such a case, like:

error: cannot delete branch '<branch_name>' because bisect is in progress