Fix exception-safety in callbacks from Rust to C++

[graydon]

We've been using cxx slightly wrong around callbacks from Rust to C++. This isn't likely a problem as we only have two such callbacks and they call into the C++ logging system which rarely if ever throws, but just to be on the safe side we should define these functions as returning cxx::Result not a plain Rust type. It turns out cxx puts a noexcept wrapper around all such callbacks to ensure they don't throw (and confuse Rust ABIs) and by doing this it means any throw in the C++ side would wind up causing the C++ runtime to call std::terminate (which is what it does when a noexcept function throws anyways).

The fix is very straightforward.