chore: avoid raise exception from within group_manager

[Ivansete-status]

Background

We need to minimize the use of "raise" exceptions.

Details

The changes should be applied in waku/waku_rln_relay/group_manager/on_chain/group_manager.nim.

• Substitute raise exception approach by Result ( valueOr, isOkOr .)

Acceptance criteria

No exceptions are raised within waku/waku_rln_relay/group_manager/on_chain/group_manager.nim

[stubbsta]

This also impacts the workflow to stop the anvil process which needs to be improved.
From discord:

So about the "orphaned anvil process" complaint seen in some of the recent CI logs (cc: @TanyaS ), I remembered that when I was working on [this PR](https://github.com/logos-messaging/nwaku/pull/3650) I noticed that if tests raise after runAnvil(), then the teardown: section you see in some of the test files (which calls stopAnvil()) doesn't get called, which leaves an anvil process running (orphaned, owned by some system process). You'd have to cover the entire span of code after runAnvil() with a try to catch the error and allow teardown to run. For example [this](https://github.com/logos-messaging/nwaku/blob/54f4ad8fa2ed452df670e25213a7fa34e5cc5432/tests/waku_rln_relay/test_wakunode_rln_relay.nim#L32) would be something like (probably doesn't need to stop in except/finally since the teardown would then run, I guess):

  setup:
    anvilProc = runAnvil()
    try:
      manager = waitFor setupOnchainGroupManager()
    except CatchableError:
      stopAnvil(anvilProc)

  teardown:
    stopAnvil(anvilProc) # <-- doesn't run if setupOnChainGroupManager() error is uncaught

Fabiana

 — 19:58
I think there's a bug in my suggestion there, it's probably except Exception:   (catch "defects" and etc.