Task could warn or error if task graph has a cycle

[alexec]

I accidentally introduce a cycle is my task graph. Rather than returning an error, it got stuck. Took me ages to understand why this happened as no diagnostics were output.

[korverdev]

I believe there's been some discussion on this here:
#820

Just wanted to link it for everyone's convenience.

[pd93]

@korverdev Thanks, I knew I'd seen another related ticket. Let's keep the conversation together in #820.