remove branch with base fails to delete local when remote is already gone

[itkoren]

Upon finish rc branch, in case the remote had already been deleted, git flux will fail deleting the remote and will not continue to delete the local branch

git flux rc finish 8.3.0.0

? local and remote branches will be deleted, forever! continue? (y/n) y

✘ branch 'rc/8.3.0.0' don't exist on the remote.