webdav: Attempt to remove failed uploads

Some webdav backends (eg rclone serve webdav) leave behind half
written files on error.  This causes the integration tests to
fail. Here we remove the file if it exists.

backend/webdav/webdav.go

@@ -968,6 +968,8 @@ func (o *Object) Update(in io.Reader, src fs.ObjectInfo, options ...fs.OpenOptio
 		return shouldRetry(resp, err)
 	})
 	if err != nil {
+		// Remove failed upload
+		_ = o.Remove()
 		return err
 	}
 	// read metadata from remote