prune --images will show many errors when multiple workdirs

[dpordomingo]

Since the same images can be used by many workdirs, sourced prune --images can show many docker-compose errors in such installations

sourced-ce/cmd/sourced/cmd/prune.go

Line 18 in 27453ad

a = append(a, "--rmi", "all")

e.g.

Removing image srcd/gitbase:v0.20.0
ERROR: Failed to remove image for service gitbase: 409 Client Error:
  Conflict ("conflict: unable to remove repository reference "srcd/gitbase:v0.20.0"
  (must force) - container 90f006083a79 is using its referenced image e6029594062e")

I see that it can be expected behavior, but I wonder if we'd like to improve the DX is these situations, like:

• if there are already prepared workdirs, skip image deletion, and notice the user about that.