Replace uses of os.IsExist and os.IsNotExist with errors.Is.

The former don't properly unwrap errors.

webserver/api.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -170,7 +171,7 @@ func apiGroupHandler(w http.ResponseWriter, r *http.Request, pth string) {
 		return
 	} else if r.Method == "PUT" {
 		etag, err := group.GetDescriptionTag(g)
-		if os.IsNotExist(err) {
+		if errors.Is(err, os.ErrNotExist) {
 			err = nil
 			etag = ""
 		} else if err != nil {
@@ -299,7 +300,7 @@ func usersHandler(w http.ResponseWriter, r *http.Request, g, pth string) {
 		return
 	} else if r.Method == "PUT" {
 		etag, err := group.GetUserTag(g, username)
-		if os.IsNotExist(err) {
+		if errors.Is(err, os.ErrNotExist) {
 			etag = ""
 			err = nil
 		} else if err != nil {

webserver/api_test.go

@@ -291,7 +291,7 @@ func TestApi(t *testing.T) {
 	}
 
 	_, err = group.GetDescription("test")
-	if !os.IsNotExist(err) {
+	if !errors.Is(err, os.ErrNotExist) {
 		t.Errorf("Group exists after delete")
 	}
 }

webserver/webserver.go

@@ -120,7 +120,7 @@ func notFound(w http.ResponseWriter) {
 var ErrIsDirectory = errors.New("is a directory")
 
 func httpError(w http.ResponseWriter, err error) {
-	if os.IsNotExist(err) {
+	if errors.Is(err, os.ErrNotExist) {
 		notFound(w)
 		return
 	}
@@ -235,7 +235,7 @@ func (fh *fileHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		ff, err := fh.root.Open(index)
 		if err != nil {
 			// return 403 if index.html doesn't exist
-			if os.IsNotExist(err) {
+			if errors.Is(err, os.ErrNotExist) {
 				http.Error(w, "Forbidden", http.StatusForbidden)
 				return
 			}