RPC: Correct behavior when enabled in config but compiled-out?

Currently, when arti starts with RPC support compiled out, it gives a warning if you try to configure RPC to be enabled.

At !2918 (comment 3186198) @Diziet suggests that it should fail to start with an error instead. See that comment for rationale.

We should figure out the right behavior here. (Please don't implement this without discussion.)