Fix min/max_exponent values (these should be base 10 not base 2 as before).

git-svn-id: http://svn.boost.org/svn/boost/trunk@86778 b8fc166d-592f-0410-95f2-cb63ce0dd405
Add some better error checks to interval construction.

git-svn-id: http://svn.boost.org/svn/boost/trunk@86779 b8fc166d-592f-0410-95f2-cb63ce0dd405