Better messages when parallel conversions in multiple tabs are complete

[iuravic]

Is your feature request related to a problem? Please describe.
Here's an example which explains the some lack of clarity and accuracy with a UI message.

Let's say there are 3 different tabs with conversions running in parallel. And let's say:

• tab 1 just finished converting batch 21/23,
• tab 2 is still converting 22/23,
• and tab 3 is also still converting 23/3.
  Presently when tab 1 finishes converting all the posts, and there it's done because there are no more batches to convert, it will show a message to the user message which says something like "there are some posts left unconverted, you can try converting these again by clicking [here]" -- while these unconverted posts are just posts still in queue in tabs 2 and 3, and not actual failed/skipped posts.

Describe the solution you'd like
The message in tab 1 when it finishes running its last conversion batch, should instead check whether other tabs are still converting, and display this kind of a message instead:

• e.g. "finished converting all posts in the conversion queue. You may close this tab now, and check your other running tabs for completion."