Retain original name of default tasklist (if possible)
When deleting all tasklists before import, or attempting to replace the contents of the default tasklist, GTI was renaming the default tasklist to "Undeletable default nnnnnnnnnn", where nnnnnnnnnn is a unique number.
Now, GTI tries to keep the original name of the default tasklist, unless the import created a new tasklist with the same name. In that case, the default tasklist will still be renamed "Undeletable default nnnnnnnnnn"
Attempt to prevent the very occasional HttpError 400 "Invalid value" error when creating a tasklist.
This appears to be due to incorrectly formatted data in the import file, which results in an apparently invalid tasklist name.
The most likely cause is missing or unpaired double-quotes, which means that the entire task is being interpreted as the tasklist name. This exceeds the maximum 256 character tasklist name length, thus causing a 400 "Invalid value" error.
Now GTI checks for tasklist name longer than 256 characters and reports that immediately to the user.