Add check for whether endpoint and endpoint.code input values were swapped

**Description**

If the `endpoint` and `endpoint.code` column names are swapped, the `get_cox_res` function will fail with an error message similar to:

```r
error in survival::coxph(formula = cox.formula, data = in.df) :
  No (non-missing) observations
```

The function should check that the values in the columns match up with what is expected.