Cancelling a TM merge does not actually cancel, prevents further merges

0. With a project version

• containing e.g. zanata-platform's messages.properties
• is reasonably translated (see translate.zanata.org)

1. Create a new version
2. upload the messages.properties to new version
3. Select TM Merge translations from the menu

4. Select

• 75% from the bands
• previous version (this project) for the source
• ignore document and context

5. Press Merge
6. Press Cancel
7. Press Merge again when the dialog shows

Expected:
New merge process begins

Actual:
The process is still active, the handle preventing any further merges until it is complete. The error given to the user is vague.

See:
https://pastebin.com/FH43n55x