[boost] boost spirit exception example

Agreed that if spirit_parsing_context was available and part of the spirit librairy, it would be ideal. However, its not, and most libraries dont have that. In those cases, it might be nice to have a wrapper to "error_info" in addition to the other throw methods that people want. I think that its probably prudent to have a few different ways to throw exceptions and not enforce any particular strategy on the user.