Non-fatal non-exception based error handling should be provided

Using exceptions is the only way to parse xml and have it not crash the program on a formatting error. So if we can't use exceptions, rapidxml becomes unusable if you want to gracefully handle bad formatting.

The usage of a globally linked error handler is also problematic, multiple places in the code using rapidxml w/o exceptions will cause linking problems, and it requires workarounds to wrap the #include <rapidxml.hpp> in a namespace so multiple error handlers can be provided.

Ideally a function pointer object could be provided to rapidxml that would allow us to do our own exception handling, that could contain user data where we can map to our own local information, so the filename and such can be printed in a thread-safe manner.

Start with rapidxml.hpp and inspect the current non-exception error-handling path and its globally linked handler. Determine how a caller-provided handler and user data could support local, thread-safe error reporting without namespace workarounds; done means malformed XML can be handled without exceptions or link conflicts.