Create a new Error enum type

[grunch]

When a user send an wrong message to mostrod, mostrod responds with a message with Action: CantDo, we have the need to give more details with the reason of this CantDo message, for this we want to create a new enum Error, this enum must have different type of errors and mostrod will send this in the payload of a message with Action: CantDo, we need to modify the payload like this:

pub enum Payload {
    Order(SmallOrder),
    PaymentRequest(Option<SmallOrder>, String, Option<Amount>),
    TextMessage(String),
    Peer(Peer),
    RatingUser(u8),
    Amount(Amount),
    Dispute(Uuid, Option<u16>),
    Error(Error),
}

[grunch]

closing via #70