Return the same error codes as provided by 3scale backend about 3scale_ws_api_for_ruby

[mpguerra]

The way the ruby plugin currently works is that the error code is not the http status code, but a 3scale error code which used to be sent as an attribute of the xml response sent and looked something like this:

<error code="application_not_found">application with id="foo" was not found</error>

This is no longer present when making calls that fail, however we need to think carefully about how to change this to ensure that this isn’t breaking anything else instead.