assertions.rb: refine message
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46568 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
assertions.rb: refine message
refine default message for the exception to be raised.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46568 b2dd03c8-39d4-4d8f-98ff-823fe69b080e