hydra-eval-jobs: Transmit original Nix error when handling aggregate jobs

It might happen that a job from the aggregate returned an error!

This is what the vague "[json.exception.type_error.302] type must be string, but is null" was all about in this instance; there was no drvPath to stringify!

So we now actively watch for errors and copy them to the aggregate job.

Created by  Samuel Dionne-Riel  on October 25, 2020
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