assertions.rb: improve assert_throw

* lib/test/unit/assertions.rb (assert_throw): revert r44379 and
  improve failure message, since assert_throws cannot extract the
  inspection from the execption message correctly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44382 b2dd03c8-39d4-4d8f-98ff-823fe69b080e