Fix the /api/v2/transaction/verify

[Senyoret1]

The /api/v2/transaction/verify API endpoint is not working as described in the documentation ( https://github.com/skycoin/skycoin/blob/develop/src/api/README.md#verify-encoded-transaction ):

• When trying to decode a transaction with inputs that are not in the unspent pool or the historical archive, the node returns an error like this: Uxout of --- does not exist.. that is not what the docs say, which is the transaction "inputs" metadata will be absent and only "uxid" will be present.. The endpoint must work as described in the docs, to be able to show a preview in the UI before signing a raw transaction in an offline computer.

• If you send an already spent transaction, the API returns 504 Gateway Timeout, instead of 422 Unprocessable Entity, as indicated in the documentarion.