raise an error when needed.base_url has not been set

Issue #41 resolved
Jan-Wijbrand Kolman
created an issue

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.