Fix #4. Detect HTTP RESPONSES different than 200 on Cursor objects

[pitbulk]

See #4

[pitbulk]

@richet what do you think? Is there a more elegant way to manage this?

TODO: Add documentation

[richet]

This doesnt actually work. If you run this branch in the console using some Auth Only credentials you would expect it to produce a 401 error but in reality it just returns nil.

e.g. Run the sdk console

> bin/console

client = OneLogin::Api::Client.new(
  client_id: 'xxx',
  client_secret:'xxx',
  region: 'us'
)

users = client.get_users
# returns the cursor but no error yet as the users not fetched until each is called

users = client.get_users.each{|u|u.id}
# returns nil and since the error is applied to the cursor and not the client it is not visible 

[pitbulk]

The master branch return no error on client too.

My patch at provides error on the cursor object but I dont know how to propagate the error to the client.