Return actual error types instead of generic + string

idiomatic go suggests you should pass an specific error type instead of just a string. For example, if i query for a ticker position and dont have one, you should return `ErrPositionNotExist`, not a string.

example, return something like this: `var ErrPositionNotExistForTicker = errors.New("position does not exist")`

then i can check with `error.Is()`