[Mercurial] Squashing a pull-request results in an error without message (BBCC-503)

There isn't that much else to say. I've added a screenshot that shows the error screen and also the network requests in the Firefox developer tools. You can clearly see the POST request for executing the merge (fulfill), followed by a GET request for the status. You can see the data of the response in the right bottom panel. complete is set to true, and the error array is empty: