Register RaiseError Middleware Under Non-Conflicting Name

This gem includes a the DeskApi::Response::RaiseError middleware, which was registered as :raise_error.  However, there is already a built-in middleware from Faraday that is registered under this same name.  This causes the built-in middleware to be silently replaced by desk's, which I have observed to break other clients that need to use :raise_error with Faraday.

To fix this, I have changed the registered name of the middleware to :raise_desk_api.