#54 SaveFile does not set error on failure

[Nicola Civran]

I'm not sure if this is a bug, but it will be nice if TiXmlDocument::SaveFile() sets a specific error message on failure, something like that:

SetError( TIXML_ERROR_SAVING_FILE, 0, 0, TIXML_ENCODING_UNKNOWN );

Now, it simply returns false, leaving all error-related variables unchanged.