fix the compile while test

lib/virtual/method_source.rb

@@ -60,7 +60,7 @@ module Virtual
     # the added instruction will become the new insertion point
     def add_code instruction
       unless (instruction.is_a?(Instruction) or instruction.is_a?(Register::Instruction))
-        raise instruction.inspect
+        raise instruction.to_s
       end
       @current.add_code(instruction) #insert after current
       self