REF: do all convert_tolerance casting inside Index.get_loc
As a bonus we get better exception messages in the affected tests.
is this actually a bug fix? e.g.the error message before was ints and not its (correctly) timestamps.? > e.g.the error message before was ints and not its (correctly) timestamps.?

I guess so, sure lgtm ex merge conflict rebased+green added whatsnew for improved exception messages, rebased+green ok!