config: wrong `cl_node_url` value leads to watchdog timeout vs explicit error

When using the new `genesis_fork_version` config option from #201 there's now nothing explcitly checking that `cl_node_url` is valid, instead of the `PayloadSourceReconnector` will spin until eventually the watchdog timeout is hit and rbuilder shuts down.

For example with this config:

```yaml
genesis_fork_version = "0x00112233"
cl_node_url = ["http://localhost:3501"] #should be 3500
watchdog_timeout_sec = 10
```

I get:

```
2024-10-02T19:49:35.807494Z INFO rbuilder::live_builder::payload_events::payload_source: PayloadSourceReconnector connecting
2024-10-02T19:49:35.808777Z ERROR rbuilder::live_builder::payload_events::payload_source: PayloadSource stopped, reconnecting
2024-10-02T19:49:35.808791Z INFO rbuilder::live_builder::payload_events::payload_source: PayloadSourceReconnector waiting to reconnect
...
2024-10-02T19:49:45.815890Z ERROR rbuilder::live_builder::watchdog: Watchdog timeout
```