[MM-28923] Fix errcheck issues in server/channels/app/file.go (#30878)

Co-authored-by: Claude <noreply@anthropic.com>

server/channels/app/file_test.go

@@ -308,7 +308,7 @@ func TestCreateZipFileAndAddFiles(t *testing.T) {
 		err := th.App.CreateZipFileAndAddFiles(&mockBackend, []model.FileData{}, zipName, directory)
 
 		require.Error(t, err)
-		require.Equal(t, err.Error(), "only those who dare to fail greatly can ever achieve greatly")
+		require.Equal(t, err.Error(), "failed to write zip file to file backend at path directory-to-heaven/zip-file-name-to-heaven.zip: only those who dare to fail greatly can ever achieve greatly")
 	})
 
 	t.Run("write no file", func(t *testing.T) {