raise an error when needed.base_url has not been set
The needed.base_url is required for computing library and resource URLs. The base_url can be set during configuration time" (when instantiating the NeededResource() class) and/or during "request time" (when the application framework is in charge of computing base URLs).
When no base_url is set at all, it seems logical to raise an error instead of assuming an empty string when computing the library/resource URLs.
That way, setting the correct base_url cannot be forgotten.