[#972] Do not error if the resource does not exist for datastore dumps
[#972] and [#628] Add docs for datastore dumps again (they have been written for #628 but removed by docs reorganisation).
[#972] After a discussion with @vitorbaptista, we decided to stick with datastore resource instead of datastore table
Merge pull request #972 from okfn/972-datastore-dump-fixes

Datastore dump raises 500 if resource does not exist