Handle common newznab errors

It seems common for newznab servers to return a 200 status with an error in the payload.

    <error code="500" description="Request limit reached"/>

This changeset handles this case by logging the code and description as an error.

Tested the error paths on nzbndx.net and nzbs4u.com. I don't have a functioning newznab account to test the positive case at the moment.