Handle 404 errors in getAggregatedAttestation response

[nflaig]

As per spec the getAggregatedAttestation could return a 404 status code if no aggregated attestation is available

apis/validator/aggregate_attestation.yaml#L11-L12

A 404 error must be returned if no attestation is available for the requested attestation_data_root.

But it looks like Nimbus does not specifically handle this case right now which would lead to an error that looks critical but are somewhat expected if Nimbus VC is connected to multiple BNs (see ChainSafe/lodestar#6634 (comment))

nimbus-eth2/beacon_chain/validator_client/api.nim

Lines 1716 to 1718 in dc19b08

	handleUnexpectedCode()
	ApiResponse[GetAggregatedAttestationResponse].err(
	ResponseUnexpectedError)

We have been getting user reports about this ChainSafe/lodestar#6631 which we already addressed on the Lodestar bn (ChainSafe/lodestar#6648) but based on how our attestation cache works Lodestar bn might not be able to serve this request.

Maybe Nimbus VC could also consider to log this differently to avoid user concerns, as based on how Nimbus handles this, if one of the connected BNs is not able to serve the request, it's not critical.

[tersec]

#6254