no prune done & no warning/alert when no stage defined

[MathRobin]

Hi ! We have met a simple problem, we use different stages for the same apps (at least 3 stages for 26 services, ~80 stages). Our stage is not described in serverless yml but in CLI calls (even in automated scripts, workflows & everything else).

But we made an error and forgot to add the stage flag (-s) in the prune CLI command (can happen).

What happened : nothing.

It cleaned nothing (of course, no stage targeted) but no warning or error has poped to tell us we are stupid.

Maybe a small message could be great to help debug rookies (and dumb like me)

Thanks for your work !