store/remote: return NotExist error on HTTPS Get
store/remote: return NotExist error on HTTPS Get

The fast path for fetching objects by HTTPS was not returning a
structured errors.NotExist error on a 404. Fix that.

Change-Id: I5fd18e2974583d802e04cbebac8ff7af66e4c9a8
Reviewed-on: https://upspin-review.googlesource.com/12020
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin