Errors handling

Team,

Changed the member handling messages to make sure we perform a present? check before doing any actions.

I also changed the i18n messages to make their key more explicit because the key is now part of the hash returned in case of failure.

Example if a subscribe call is made on a user that does not belong to the list:
`{:status=>"failure", :error=>{:http_status_code=>"", :code=>"record_not_found", :message=>"The member has not been found in the list"}}`

Any comments appreciated 

Thank you
That code looks good to me. For this question:
<blockquote>Another question that is not related to this but makes me think about what if the riid and the email do not match the same record? Should we ensure that the values given are correct?</blockquote>

It would be over engineering to code for that scenario. The gem's job is to facilitate the ability to talk to the Responsys Interact API through Ruby, but it's the client's (ex: thredUP) responsibility to worry about data integrity. If someone did something to alter a RIID for a customer, then they should be on the hook for the repercussions of that action.