REF: disambiguate get_loc_level k variable
2 logic-neutral prelims split off from other branches

1) instead of catching and re-issuing TypeError in NDFrame.xs, just issue it with the correct exception message in the first place
2) inside _get_loc_level there is a 'k' variable that is serving a dual purpose as a loop index and as a variable inside that loop.  AFAICT this is for a small code-de-duplication in checking for slices, but it turns out we need to treat slices differently in the two cases.  This changes the inside variable to `loc_level`