Handle invalid operations on merged datastore endpoint

- Move the other endpoints under the same thats handling the get
- Add exceptions to handle invalid cases
- Add new tests for testing the invalid cases

Issue-ID: CPS-1001
Issue-ID: CPS-1219
Change-Id: I26fd57b5921f976cde7089b4cf1e8c0e5d8dc43b
Signed-off-by: bmiklos <miklos.baranyak@est.tech>
8 files changed