slightly better messages for unsupported ruby

Very slightly unfortunately, just just enought to find the error.
Also testing what is unsupported, so it's documented.

test/ruby/test_not_implemented.rb

@@ -0,0 +1,20 @@
+require_relative "helper"
+
+module Ruby
+  class NotImplemented < Minitest::Test
+    include RubyTests
+
+    def assert_handler_missing(input)
+      assert_raises(Ruby::ProcessError){ compile(input) }
+    end
+
+    def test_module
+      assert_handler_missing "module Name ; end"
+    end
+
+    def test_module_module_scoped
+      assert_handler_missing( "M::Module" )
+    end
+
+  end
+end