feat: add failed submission tasks to task management

[ninsch3000]

When submitting a demo task to an unavailable instance, an error banner appears on the submission page, but the task is not added to the task management overview. Is this desired?

Could it maybe be useful to add those tasks to the overview?

PRO: a user can see at a glance their failed submission attempts. Like that they could remember unavailable or wobbly instances, and also all attempts would be logged in one place.

CON: Overcrowding of task management overview?

[ninsch3000]

discussion: Do NOT add tasks that fail to be submitted.