Propagate errors when some tasks in BgTask fail

[jopemachine]

Motivation  

• As in fix(BA-693): Failure of whole image rescanning when there is a misconfigured container registry #3652, when all tasks in BgTask are successfully executed but some included tasks fail, a mechanism is needed to propagate this information to the client.

Required Features

• Since the entire BgTask is successfully completed, BgTaskDone will be published.
• So, it will be necessary to either add an additional field to BgTaskDone to include information about partial failures or define a separate event to convey the partial failures.

Impact

• Whether adding a new field to BgTaskDone or publishing a new type of event, there should be no breaking changes.

Testing Scenarios  

• Test the partial failure scenario like fix(BA-693): Failure of whole image rescanning when there is a misconfigured container registry #3652.