dont skip keyerror for IntervalIndex
cc @jschendel this is speculative that this is the desired behavior.

This is our only use of `Index.is_interval()`
no tests changed? or just running a previously skipped a skipped test? rebase, lgtm. > no tests changed?

No tests affected.  Found this while trying to shave off corner cases in indexing, and it isn't clear that this is needed. lgtm. merge on green.