kernel-install: if a plugin fails, return error immediately

kernel-install: if a plugin fails, return error immediately

Since the first version in 81516adcb71a47837544340f72eb8ee810274119,
kernel-install would "gather" a return value by summing the exit codes
of the plugins… This makes no sense, because those are not additive values.

Let's just break off immediately. We now implement cleanup via trap, so if we
break, we should leave no garbage behind.