start: don't return false when the container's init exits nonzero

start: don't return false when the container's init exits nonzero

This seems slightly counter-intuitive, but IMO it's what we want.
Basically, ->start() should succeed if the container is spawned correctly
(similar to how golang's exec.Cmd.Start() returns nil if the thing spawns
correctly), and users can check error_num (i.e. golang's exec.Cmd.Wait())
to see how it exited.

This preserves previous behavior, which basically was that start was always
successful if the thing actually launched. Since we never kept track of
exit codes, this would always succeed too. Now that we do, it doesn't, and
this change is required.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>