Make client return with a response object

[roberthoner]

I think we should stop raising exceptions for every non-20x response. Instead, we should only be raising exceptions for server errors (50x) and timeouts.

If the downstream service returns a validation error or some other 40x error, it should be up to the consuming service to handle this gracefully. The only thing it doesn't have to handle gracefully on its own are timeouts and 50x errors.

To facilitate this, the Rester::Client should return a response object like so:

responce = Service.resources(id).get

if response.successful?
  response[:key] # => access response data as hash keys.
else
  response[:error] # => type of error
  response[:message]
end