libhfds++: C++ exceptions should never escape the C API

Some low-level subsystems (asio, rapidxml) throw exceptions on error. While our internal code doesn't utilize exceptions, these are part of the libraries we are using.

Calls to asio should be wrapped in try blocks.

For additional safety, all of the C api implementations should have try...catch(...) blocks to catch any C++ exceptions and make sure they don't bring down the calling stack, which might have no concept of exceptions.