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
Think that's a good idea since if we have the riid we should point to the right user.

```ruby
def present?(list)
      if !@user_riid.nil?
        response = lookup_via_primary_key(list, "RIID", @user_riid)  
      else
        response = lookup_via_primary_key(list, "EMAIL_ADDRESS", @email)  
      end
      
      !(response[:status] == "failure" && response[:error][:code] == "RECORD_NOT_FOUND")
    end
```

Is that okay ?

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 ?