Krikri::SpawnError

Raised - only for callers that opt in via raise_spawn_errors: - when a process could not be spawned at all: pipe setup, argv parsing or Process.new failed before the child ever ran. Distinct from a process that ran and exited nonzero, which keeps returning its real exit code (#704: a spawn failure used to masquerade as exit 1 / 255, and plugins branching on that code - e.g. sysctl probing a live value - took silently wrong branches and reported success).