Add proper handling for submit blind block 502 error (#15848)

* Add proper handling for builder relay 502 BadGateway errors

* James feedback

* Change wording

validator/client/beacon-api/beacon_api_validator_client_test.go

@@ -202,10 +202,10 @@ func TestBeaconApiValidatorClient_ProposeBeaconBlockError_ThenPass(t *testing.T)
 
 func TestBeaconApiValidatorClient_ProposeBeaconBlockAllTypes(t *testing.T) {
 	tests := []struct {
-		name          string
-		block         *ethpb.GenericSignedBeaconBlock
-		expectedPath  string
-		wantErr       bool
+		name         string
+		block        *ethpb.GenericSignedBeaconBlock
+		expectedPath string
+		wantErr      bool
 		errorMessage string
 	}{
 		{
@@ -374,7 +374,7 @@ func TestBeaconApiValidatorClient_ProposeBeaconBlockHTTPErrors(t *testing.T) {
 				gomock.Any(),
 				gomock.Any(),
 			).Return(nil, nil, tt.sszError).Times(1)
-			
+
 			if tt.expectJSON {
 				// When SSZ fails, it falls back to JSON
 				jsonRestHandler.EXPECT().Post(

validator/client/beacon-api/mock/json_rest_handler_mock.go

@@ -121,7 +121,7 @@ func (m *MockJsonRestHandler) PostSSZ(ctx context.Context, endpoint string, head
 	ret0, _ := ret[0].([]byte)
 	ret1, _ := ret[1].(http.Header)
 	ret2, _ := ret[2].(error)
-	return ret0,ret1,ret2
+	return ret0, ret1, ret2
 }
 
 // Post indicates an expected call of Post.