fix incorrect config validation regression from #5959 (#5966)

During refactoring of #5959, some implicit `return` were overlooked,
resulting in spurious `err()` being returned without message.

```
{"lvl":"WRN","ts":"2024-02-26 10:12:20.469+00:00","msg":"Beacon nodes report different configuration values","reason":"","service":"fallback_service","node":"http://127.0.0.1:9303[Nimbus/v24.2.1-4e9bc7-stateofus]","node_index":0,"node_roles":"AGBSDT"}
```

Correcting the helpers to return explicit result in all exhaustive
cases so that this cannot happen anymore by accident.

beacon_chain/validator_client/common.nim

@@ -1440,23 +1440,24 @@ proc updateRuntimeConfig*(vc: ValidatorClientRef,
         localForkEpoch: Epoch,
         forkVersion: Opt[Version]): Result[void, string] =
       if localForkVersion.isNone():
-        discard  # Potentially discovered new fork, save it at end of function
+        ok()  # Potentially discovered new fork, save it at end of function
       else:
         if forkVersion.isSome():
           if forkVersion.get() == localForkVersion.get():
-            discard  # Already known
+            ok()  # Already known
           else:
-            return err("Beacon node has conflicting " &
-                       consensusFork.forkVersionConfigKey() & " value")
+            err("Beacon node has conflicting " &
+                consensusFork.forkVersionConfigKey() & " value")
         else:
           if wallEpoch < localForkEpoch:
             debug "Beacon node must be updated before fork activates",
                   node = node,
                   consensusFork,
                   forkEpoch = localForkEpoch
+            ok()
           else:
-            return err("Beacon node must be updated and report correct " &
-                       $consensusFork & " config value")
+            err("Beacon node must be updated and report correct " &
+                $consensusFork & " config value")
 
     ? ConsensusFork.Capella.validateForkVersionCompatibility(
       localForkConfig.capellaVersion,
@@ -1468,23 +1469,24 @@ proc updateRuntimeConfig*(vc: ValidatorClientRef,
         localForkEpoch: Epoch,
         forkEpoch: Epoch): Result[void, string] =
       if localForkEpoch == FAR_FUTURE_EPOCH:
-        discard  # Potentially discovered new fork, save it at end of function
+        ok()  # Potentially discovered new fork, save it at end of function
       else:
         if forkEpoch != FAR_FUTURE_EPOCH:
           if forkEpoch == localForkEpoch:
-            discard  # Already known
+            ok()  # Already known
           else:
-            return err("Beacon node has conflicting " &
-                       consensusFork.forkEpochConfigKey() & " value")
+            err("Beacon node has conflicting " &
+                consensusFork.forkEpochConfigKey() & " value")
         else:
           if wallEpoch < localForkEpoch:
             debug "Beacon node must be updated before fork activates",
                   node = node,
                   consensusFork,
                   forkEpoch = localForkEpoch
+            ok()
           else:
-            return err("Beacon node must be updated and report correct " &
-                       $consensusFork & " config value")
+            err("Beacon node must be updated and report correct " &
+                $consensusFork & " config value")
 
     ? ConsensusFork.Altair.validateForkEpochCompatibility(
       localForkConfig.altairEpoch, forkConfig.altairEpoch)