Trades without results gives an error

[nikhilsaraf]

When a trade has no results, it gives an error

{
  "type": "https://stellar.org/horizon-errors/not_found",
  "title": "Resource Missing",
  "status": 404,
  "detail": "The resource at the url requested was not found.  This usually occurs for one of two reasons:  The url requested is not valid, or no data in our database could be found with the parameters provided."
}

this should be handled correctly by:

1. checking if both assets exists
2. if both exist then return a result indicating that there are no trades
3. if at least one asset does not exist then it is ok to return the original error

Referenced issue in stellar/go: stellar/go#2280