Replace "Unknown" progress label in background migrations UI

Jon Jenkinsrequested to merge
597883-bbm-unknown-update into master

What does this MR do and why?

Replace "Unknown" progress label in background migrations UI with clearer, more helpful message

Screenshots or screen recordings

Before After
image image image

How to set up and validate locally

  1. Create a row for a batched background migration with a NULL value in total_tuple_count in the table batched_background_migrations.
  2. On master, the old Unknown will be displayed. On this branch, the new message will be displayed.
  3. Mouse over the new message for more details.

Related to #597883 (closed)

Edited by Jon Jenkins

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