Make use of Chris Dembia's new NiceTypeName::namestr() method

Simbody used `SimTK::NiceTypeName::name()` for some error messages, but those type names come out mangled on some compilers. @chrisdembia added a `namestr()` method in PR #223 that returns demangled names on all platforms.

This PR makes some minor Simbody changes to exploit the availability of the new method. I also expanded some comments and made one probably-unnecessary code safety change to `demangle()`.