fix(flashblocks): gracefull cancellation on payload build failure

[julio4]

📝 Summary

Close #209

Within a payload building job in flashblocks mode, we sequentially build flashblocks and a timer task coordinate timing of building these flashblocks. When a flashblock building fails, the whole payload building job fails and we need to gracefully shutdown all tasks related to this job. This is implemented by cancelling the payload building job cancellation token on flashblock building error, which correctly signal to the associated timer task to shutdown itself.

• Also fix channel variable names

💡 Motivation and Context

The channel between timer task to coordinate flashblock building was not gracefully shutdown when payload building fail, causing unwanted error and logs.

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✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[SozinM]

I think with this we still won't get insight into the error we got

[julio4]

I think with this we still won't get insight into the error we got

We do log the error here