REF: disambiguate get_loc_level k variable

[jbrockmendel]

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