Fix error handling in Router.Walk

In old version error was not returned properly, In one walkFn call
error was checked only for SkipRouter but not for nil.

mux.go

@@ -285,6 +285,9 @@ func (r *Router) walk(walkFn WalkFunc, ancestors []*Route) error {
 		if err == SkipRouter {
 			continue
 		}
+		if err != nil {
+			return err
+		}
 		for _, sr := range t.matchers {
 			if h, ok := sr.(*Router); ok {
 				err := h.walk(walkFn, ancestors)

mux_test.go

@@ -5,6 +5,7 @@
 package mux
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 	"strings"
@@ -1195,6 +1196,42 @@ func TestWalkNested(t *testing.T) {
 	}
 }
 
+func TestWalkErrorRoute(t *testing.T) {
+	router := NewRouter()
+	router.Path("/g")
+	expectedError := errors.New("error")
+	err := router.Walk(func(route *Route, router *Router, ancestors []*Route) error {
+		return expectedError
+	})
+	if err != expectedError {
+		t.Errorf("Expected %v routes, found %v", expectedError, err)
+	}
+}
+
+func TestWalkErrorMatcher(t *testing.T) {
+	router := NewRouter()
+	expectedError := router.Path("/g").Subrouter().Path("").GetError()
+	err := router.Walk(func(route *Route, router *Router, ancestors []*Route) error {
+		return route.GetError()
+	})
+	if err != expectedError {
+		t.Errorf("Expected %v routes, found %v", expectedError, err)
+	}
+}
+
+func TestWalkErrorHandler(t *testing.T) {
+	handler := NewRouter()
+	expectedError := handler.Path("/path").Subrouter().Path("").GetError()
+	router := NewRouter()
+	router.Path("/g").Handler(handler)
+	err := router.Walk(func(route *Route, router *Router, ancestors []*Route) error {
+		return route.GetError()
+	})
+	if err != expectedError {
+		t.Errorf("Expected %v routes, found %v", expectedError, err)
+	}
+}
+
 func TestSubrouterErrorHandling(t *testing.T) {
 	superRouterCalled := false
 	subRouterCalled := false