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
The parentheses are unnecessary. It can be:
```ruby
unless name.is_a? String && value.is_a? String
```
Why is this check there? Would `name` or `value` ever not be a string if their default value is a string?