[boost] boost spirit exception example

From: Tom Brinkman (reportbase_at_[hidden])
Date: 2007-10-05 21:44:22

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.