fix AsgServiceConfig/AnaAlgorithmConfig python unit test failure

Those tests were broken by !42411 (merged), as noted in ATLASG-1589. This change should fix that.

The specific problem was that the C++ code now does its own checks whether subtools are defined, and throws its own exceptions. Those differ from the exceptions python throws and the unit test expects. There are several ways to fix this, but it seemed most reasonable to first do the checks in python before trying to register subtools in the C++ code.

Adding the urgent label, as this currently shows up as a failure in nightly and CI tests.