Relayer can still crash upon startup due to bad RPCs

[tkporter]

Problem

Despite most relayer crashes being fixed (the ones to do with contract sync building here #4811), it's still possible to fail e.g. here

hyperlane-monorepo/rust/main/agents/relayer/src/relayer.rs

Lines 138 to 143 in 58425a2

	let mailboxes = settings
	.build_mailboxes(settings.destination_chains.iter(), &core_metrics)
	.await?;
	let validator_announces = settings
	.build_validator_announces(settings.origin_chains.iter(), &core_metrics)
	.await?;

Solution

We probably don't want to take down everything if one chain is having issues