error.c: Exception#cause

* error.c (exc_cause): captured previous exception.
* eval.c (make_exception): capture previous exception automagically.
  [Feature #8257]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43636 b2dd03c8-39d4-4d8f-98ff-823fe69b080e