Add HTTP status code as `code` to error object

It's not very good to rely on `error.message`, which is string like `Not Found`, to handle specific errors, so it would be really nice to have an access to the actual HTTP status code. 

That also means the documentation for `REST API` should be improved by adding information about available HTTP status codes for each API call and their meanings. For instance I expected `http://api.quickblox.com/geodata/find.json` to return an empty array even if nothing has been found, but in reality it responds with `404 Not Found`.