bail on missing key

```
perl> $stores[0]->delete()

Use of uninitialized value in subroutine entry at /usr/local/booking-perl/5.18.2/site/lib/ShardedKV/Continuum/Jump.pm line 23.

Use of uninitialized value $key in concatenation (.) or string at /usr/local/booking-perl/5.18.2/site/lib/ShardedKV/Storage/Rest.pm line 45.

Use of uninitialized value $key in concatenation (.) or string at /usr/local/booking-perl/5.18.2/site/lib/ShardedKV/Storage/Rest.pm line 45.

-> 0;
```

This actually issues a DELETE request for '/', which under certain configurations, may result in data loss.