Failing unwrapped scripts are executed twice

For backward compatibility purposes, following script wrapping mess in previous GM releases, we're left with the following code:

      if (!script.unwrap)
        scriptSrc = "(function(){"+ scriptSrc +"})()";
      if (!this.evalInSandbox(scriptSrc, url, sandbox, script) && script.unwrap)
        this.evalInSandbox("(function(){"+ scriptSrc +"})()",
                           url, sandbox, script); // wrap anyway on early return

In other words, if an unwrapped script fails to execute properly, it is wrapped and executed again. This might result in a page being (at least partly) modified twice.

Scripts failing because of @unwrap issues should be fixed, not "hot-patched" like this.