Use WriteStateFetchError in API handlers whenever possible (#16140)

**What type of PR is this?**

Other

**What does this PR do? Why is it needed?**

Calls to `Stater.StateBySlot` and `Stater.State` should be followed by
`shared.WriteStateFetchError` to provide the most robust error handling.

**Acknowledgements**

- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

beacon-chain/rpc/eth/rewards/handlers.go

@@ -228,7 +228,7 @@ func (s *Server) attRewardsState(w http.ResponseWriter, r *http.Request) (state.
 	}
 	st, err := s.Stater.StateBySlot(r.Context(), nextEpochEnd)
 	if err != nil {
-		httputil.HandleError(w, "Could not get state for epoch's starting slot: "+err.Error(), http.StatusInternalServerError)
+		shared.WriteStateFetchError(w, err)
 		return nil, false
 	}
 	return st, true

beacon-chain/rpc/eth/validator/handlers.go

@@ -911,7 +911,7 @@ func (s *Server) GetAttesterDuties(w http.ResponseWriter, r *http.Request) {
 
 	st, err := s.Stater.StateBySlot(ctx, startSlot)
 	if err != nil {
-		httputil.HandleError(w, "Could not get state: "+err.Error(), http.StatusInternalServerError)
+		shared.WriteStateFetchError(w, err)
 		return
 	}
 
@@ -1030,7 +1030,7 @@ func (s *Server) GetProposerDuties(w http.ResponseWriter, r *http.Request) {
 	if requestedEpoch < currentEpoch {
 		st, err = s.Stater.StateBySlot(ctx, epochStartSlot)
 		if err != nil {
-			httputil.HandleError(w, fmt.Sprintf("Could not get state for slot %d: %v ", epochStartSlot, err), http.StatusInternalServerError)
+			shared.WriteStateFetchError(w, err)
 			return
 		}
 	} else {
@@ -1181,7 +1181,7 @@ func (s *Server) GetSyncCommitteeDuties(w http.ResponseWriter, r *http.Request)
 	}
 	st, err := s.Stater.State(ctx, []byte(strconv.FormatUint(uint64(slot), 10)))
 	if err != nil {
-		httputil.HandleError(w, "Could not get sync committee state: "+err.Error(), http.StatusInternalServerError)
+		shared.WriteStateFetchError(w, err)
 		return
 	}
 
@@ -1327,7 +1327,7 @@ func (s *Server) GetLiveness(w http.ResponseWriter, r *http.Request) {
 		}
 		st, err = s.Stater.StateBySlot(ctx, epochEnd)
 		if err != nil {
-			httputil.HandleError(w, "Could not get slot for requested epoch: "+err.Error(), http.StatusInternalServerError)
+			shared.WriteStateFetchError(w, err)
 			return
 		}
 		participation, err = st.CurrentEpochParticipation()

changelog/radek_use-statefetch-error.md

@@ -0,0 +1,3 @@
+### Ignored
+
+- Use `WriteStateFetchError` in API handlers whenever possible.