fix(fleetctl): Use os.Exit instead of syscall.Exit
fix(fleetctl): exit 1 on failures in debug-info
refactor(fleetctl): Return codes to outer-most levels
refactor(fleetctl): Drop unnecessary return statement
fix(fleetctl): exit 1 in start on failure
fix(fleetctl): exit 1 in submit on failure
fix(fleetctl): exit 1 in verify on failure