Type errors for undefined functions can contain rewritten vars

[tsandall]

Example:

$ opa eval 'a := "x"; data[a](1)'
{
  "errors": [
    {
      "message": "undefined function data[__localq0__]",
      "code": "rego_type_error",
      "location": {
        "file": "",
        "row": 1,
        "col": 11
      }
    }
  ]
}

The error contains a rewritten var __localq0__ when it should contain the user-defined var a. This is probably just a matter if doing a lookup on the rewritten var map when the error is raised.