[boost] [review] Exception - result

I'm pleased to announce that the Exception library has been accepted
into boost. Thanks to all formal and informal reviewers and to Emil
Dotchevski for the library!

Summary of possible improvements:
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Design
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Nearly all reviewers made suggestions for syntactic improvements. The
"tag(value) syntax" (as suggested by John Torjo) for instance provides
sufficient syntactic sugar and seems lightweight to implement.

Suggested features in this area are (those are implementable on top of
the current design - but will keep users from reinventing the wheel if
provided by the library):
- Support for grouping information, and
- support for generating stack traces.

I personally still have a bad feeling about 'enable_error_info' - let's
just revisit and discuss it on the list before changing
boost::throw_exception.

The related discussions about clone/rethrow support is still ongoing.

Documentation
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Best practices for the catch-site should be clarified in the