[ruby/mmtk] Exit with error message if MMTK_PLAN is invalid
https://github.com/ruby/mmtk/commit/79ce2008a3
[ruby/mmtk] Exit with error message if MMTK_PLAN is invalid
https://github.com/ruby/mmtk/commit/79ce2008a3