handler: use gqlerrors.FormattedError for consistency

Author: chris-ramon

handler.go

@@ -10,6 +10,7 @@ import (
 	"github.com/graphql-go/graphql"
 
 	"context"
+
 	"github.com/graphql-go/graphql/gqlerrors"
 )
 
@@ -28,7 +29,7 @@ type Handler struct {
 	playground       bool
 	rootObjectFn     RootObjectFn
 	resultCallbackFn ResultCallbackFn
-	formatErrorFn    func(err error) gqlerrors.FormattedError
+	formatErrorFn    func(err gqlerrors.FormattedError) gqlerrors.FormattedError
 }
 
 type RequestOptions struct {
@@ -140,10 +141,10 @@ func (h *Handler) ContextHandler(ctx context.Context, w http.ResponseWriter, r *
 	}
 	result := graphql.Do(params)
 
-	if formatErrorFn := h.formatErrorFn; formatErrorFn != nil {
+	if formatErrorFn := h.formatErrorFn; formatErrorFn != nil && len(result.Errors) > 0 {
 		formatted := make([]gqlerrors.FormattedError, len(result.Errors))
-		for i, err := range result.Errors {
-			formatted[i] = formatErrorFn(err.OriginalError())
+		for i, formattedError := range result.Errors {
+			formatted[i] = formatErrorFn(formattedError)
 		}
 		result.Errors = formatted
 	}
@@ -202,7 +203,7 @@ type Config struct {
 	Playground       bool
 	RootObjectFn     RootObjectFn
 	ResultCallbackFn ResultCallbackFn
-	FormatErrorFn    func(err error) gqlerrors.FormattedError
+	FormatErrorFn    func(err gqlerrors.FormattedError) gqlerrors.FormattedError
 }
 
 func NewConfig() *Config {