Accomodate error code along with error string in api

[mascot6699]

Hi,

While using the 'trade' api many times the order is not fulfilled as the order for which it was signed is no longer valid or is partially filled and the returns 400 status code.
As the string may change I don't want to rely upon it can we have error code to sent in the apis too as a improvement?

Example response:

{
    "error": "The order you are trying to match has been partially or fully matched. Please refresh to clear it from the orderbook."
}
and
{
    "error": "Order no longer available."
}