Fix PeriodIndex.get_indexer with non-PI
I'm _pretty_ sure that for non-comparable we should be returning an array of -1s, not raising.  Can you double-check me on that @jreback?

> was this not tested before?

Most of the existing tests pass a PeriodIndex.  There is one existing test that passes a DatetimeIndex to PeriodIndex.get_indexer_nonunique (which currently incorrectly calls asi8 before doing a freq check). k thanks