hash.c: key name in error message

* hash.c (env_fetch): Add key name to message on ENV.fetch KeyError,
  as well as Hash#fetch.  [ruby-core:56062] [Feature #8649]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42025 b2dd03c8-39d4-4d8f-98ff-823fe69b080e