Use NewHTTPError to return better errors

That function is merged upstream, but the current version still depends
on some stuff not yet merged into upstream go-webdav.

storage/filesystem.go

@@ -14,6 +14,7 @@ import (
 	"path"
 	"path/filepath"
 	"regexp"
+	"strings"
 
 	"github.com/emersion/go-ical"
 	"github.com/emersion/go-vcard"
@@ -100,12 +101,19 @@ func (b *filesystemBackend) safeLocalPath(homeSetPath string, urlPath string) (s
 	dir, file := path.Split(urlPath)
 	// only accept resources in prefix, no subdirs for now
 	if dir != homeSetPath {
-		return "", fmt.Errorf("invalid request path %s", urlPath)
+		if strings.HasPrefix(dir, homeSetPath+"/") {
+			err := fmt.Errorf("invalid request path: %s", urlPath)
+			return "", webdav.NewHTTPError(400, err)
+		} else {
+			err := fmt.Errorf("Access to resource outside of home set: %s", urlPath)
+			return "", webdav.NewHTTPError(403, err)
+		}
 	}
 	// only accept simple file names for now
 	if !validFilenameRegex.MatchString(file) {
-		fmt.Printf("%s does not match regex!\n", file)
-		return "", fmt.Errorf("invalid file name")
+		debug.Printf("%s does not match regex!\n", file)
+		err := fmt.Errorf("invalid file name: %s", file)
+		return "", webdav.NewHTTPError(400, err)
 	}
 
 	// dir (= homeSetPath) is already included in path, so only file here
@@ -264,6 +272,9 @@ func (b *filesystemBackend) GetAddressObject(ctx context.Context, objPath string
 
 	info, err := os.Stat(localPath)
 	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return nil, webdav.NewHTTPError(404, err)
+		}
 		return nil, err
 	}
 
@@ -514,7 +525,7 @@ func (b *filesystemBackend) GetCalendarObject(ctx context.Context, objPath strin
 	if err != nil {
 		if errors.Is(err, fs.ErrNotExist) {
 			debug.Printf("not found: %s", localPath)
-			return nil, nil
+			return nil, webdav.NewHTTPError(404, err)
 		}
 		return nil, err
 	}