[datastore] Add IsErrNoSuchEntity in datastore/errors.go
[datastore] Add IsErrNoSuchEntity in datastore/errors.go

This was an unsurprisingly common thing to do.
Also datastore already imports errors, so this was the most logical place to put this.

Change-Id: I11f5ef777733de85cbd2a9616beab6500a47bf06
Reviewed-on: https://chromium-review.googlesource.com/1043331
Commit-Queue: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go