Why bother? I would call this user error and not even following basic APIs because the function must return dicts

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Yes. That is user error. Of course, user should return dicts.
However, If user mistake and raise exception, It is better to show kind error message (like Traceback) instead of ambiguous message like: Scanning posts...Scanning posts...Scanning posts...Scanning posts...Scanning posts...Scanning posts...Scanning posts...ERROR: render_site. Task dependency 'generate_rss' does not exist..
Would you agree with this view?

It's early and I have had no coffee yet, but read_metadata can raise any other exception, and then the exact same thing will happen, right? This would only fix one specific problem.

No. read_metadata() can raise other exception like RuntimeError, KeyError, NameError, or something, but If those were raised, Nikola show Traceback correctly. Only StopIteration make false report as far as I know.

@ralsina, opinions?

@Kwpolska I'll have to take a real look at the code that calls that. I suspect it's not a problem with every plugin but with task plugins because we are unrolling iterators, so StopIteration makes everyone go nuts.