Raise PlayerNotFound exception if player not found

Right now there is no check to ensure the response is valid and the user actually exists. The only way to find out now is to do something like:

```ruby
player = Dribbble::Player.find('garrettb')
player.id # => raises NoMethodError
```

Not a good control flow when determining if the user is valid or not. This now will raise a new Dribbble::PlayerNotFound exception if the response is a 404.

```ruby
Dribbble::Player.find('garrettb') # => raises Dribbble::PlayerNotFound
```