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_basic_values.rb

@@ -33,8 +33,8 @@ module Ruby
       assert_equal SymbolConstant , lst.class , lst.inspect
     end
     def test_dstr
-      assert_raises RuntimeError do
-        compile( '"dstr#{self}"')
+      assert_raises Ruby::ProcessError do
+        compile( '"interpolate #{self}"')
       end
     end