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

[ryanschneider]

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:

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