Fix nil pointers by returning errors instead of logging

[thomaspanf]

This PR resolves a handful of nil pointers caused by missing fmt.Errorf statements, allowing the task to gracefully exit if the beacon node goes offline.

2026/02/24 21:35:57 WARNING: Primary Beacon client disconnected (Could not get beacon block data: Get "http://100.68.178.27:5052/eth/v2/beacon/blocks/2448518": dial tcp 100.68.178.27:5052: i/o timeout), using fallback...
An error occurred while searching for the withdrawn balance: no Beacon clients were ready
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x135f6ad]

goroutine 1859 [running]:
github.com/rocket-pool/smartnode/rocketpool/watchtower.(*submitNetworkBalances).getMegapoolBalanceDetails(0xc0008382a0, {0x39, 0x92, 0x10, 0xe2, 0x72, 0xd3, 0xa9, 0x61, 0x0, ...}, ...)
	/home/tpan/dev/smartnode/rocketpool/watchtower/submit-network-balances.go:543 +0x56d
github.com/rocket-pool/smartnode/rocketpool/watchtower.(*submitNetworkBalances).getNetworkBalances.func2()
	/home/tpan/dev/smartnode/rocketpool/watchtower/submit-network-balances.go:398 +0x1c8
golang.org/x/sync/errgroup.(*Group).Go.func1()
	/home/tpan/go/pkg/mod/golang.org/x/sync@v0.16.0/errgroup/errgroup.go:93 +0x50
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 1647
	/home/tpan/go/pkg/mod/golang.org/x/sync@v0.16.0/errgroup/errgroup.go:78 +0x95

Steps to reproduce:

1. Run the balance submissions watchtower task
2. Disconnect your beacon node while getMegapoolBalanceDetails is looping over every megapool validator

A similar issue is present in a handful of other spots:

• dissolveInvalidCredentialValidators tries to print validatorFromState.Index, which doesn't exist if the GetValidatorStatus call fails.
• createFinalBalanceProof allows the task to continue even if the GetWithdrawalProofForSlot call fails.