Fly command: warns if no stalled workers are present.
Fly command: warns if no stalled workers are present.

Executing `fly prune-worker --all-stalled` should not
return an error if no stalled workers are present to
be more friendly toward automated systems.

Prints instead a warning and exits gracefully.

Signed-off-by: Alessandro Degano <55cacd5b504daf5c7cafa0b6b8e4521b86a643c4@pix4d.com>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse