feat: add pallets/on-going-referenda endpoint (#1471)

* feat: add pallets/on-going-referenda endpoint

* updated docs

* error handling when call not available
- added also a test for that case

* added test that returns referenda (call available)

* replaced unnecessary Promise.all

* small but nice improvements
- added block height in the error msg (Tarik's review)
- added a Promise.all (Filippo's review)