Throw a more descriptive error if no message is provided when validating an ed25519 condition/fulfillment

[emschwartz]

Found by @inacali in #51

If you run the following and leave out the message from the second to last line you'll see a ValidationError: Invalid ed25519 signature error. It should instead tell you that you need to pass in a message.

const cc = require('five-bells-condition')
const ed25519 = require('ed25519')

const seed =  Buffer.from('833fe62409237b9d62ec77587520911e9a759cec1d19755b7da901b96dca3d42', 'hex')
const keypair = ed25519.MakeKeypair(seed)
const edff = new cc.Ed25519()
edff.setPublicKey(keypair.publicKey)
const condition = edff.getConditionUri()
const message = new Buffer('Hello World! Conditions are here!')
edff.sign(message, seed)
const fulfillment = edff.serializeUri()
const valid = cc.validateFulfillment(fulfillment, condition, message)
console.log('valid', valid)

[emschwartz]

I just realized that it's throwing this error because it's validating against the empty buffer. For something like a signature condition type I think it should probably require that you pass a proper message. It seems likely that it would be a bug causing the user to request validating a signature on no message.