fix: do things a bit less safely during launch

Ironfan swallows a lot of errors when launching. This makes it swallow
fewer. The errors are still swallowed by Ironfan.parallel, so 'cluster
launch' has been updated to exit with non-zero status in that case.

Change-Id: I31952e469d3b6d8d801af974593e960a428b8c0a