* ext/psych/lib/psych/exception.rb: there should be only one exception
  base class. Fixes tenderlove/psych #125
* ext/psych/lib/psych.rb: require the correct exception class
* ext/psych/lib/psych/syntax_error.rb: ditto
* ext/psych/lib/psych/visitors/to_ruby.rb: ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40139 b2dd03c8-39d4-4d8f-98ff-823fe69b080e