Do not throw in destructor of ManagerBase during stack unwinding

[gassmoeller]

Noticed while working on #6097. If we throw an exception while parsing the parameters of the manager class the size of plugin_names and plugin_objects may not be the same. But if we throw an exception during the stack unwinding the program crashes without helpful error message. So only throw if during a regular destruction the size does not match.

[bangerth]

Ha, that's a wild failure mode!