Enforce L1AttributesTx in adapters

[NiloCK]

In #148, AdaptCosmosTxsToEthTxs was altered to return an error if deposit tx bytes fail to unmarshal as an ethtypes.Transaction.

We should follow up with a specific check that an L1AttributesTx exists, and this check should apply on each adapter (eth->cosmos, cosmos->eth).

[NiloCK]

Each adapter currently errors if there are no deposit txs:

(PayloadTxToCosmosTx)

monomer/adapters.go

Lines 38 to 40 in bd24e9a

	if numDepositTxs == 0 {
	return nil, errL1AttributesNotFound
	}

(CosmosTxsToEthTxs)

monomer/adapters.go

Lines 107 to 109 in bd24e9a

	if len(ethTxsBytes) == 0 {
	return nil, errL1AttributesNotFound
	}

But these checks implicitly assume that any collection of deposit txs will have an L1AttributesTx at the first position.