Workaround impossible/error alternatives for HPC report

PR input-output-hk/ouroboros-network#3920 added an HPC report. We realized that impossible case alternatives are inflating the amount of red in the report.

This issue is to consider how to mitigate that. We expect there will be some prior art within the Haskell community. EG Section 6.4 of the [HPC paper](https://ku-fpg.github.io/files/Gill-07-Hpc.pdf) by Gill and Runciman discusses impossible cases (we haven't read it yet; it might just be a lamentation).

Our other initial idea was to somehow introduce an impossible case combinator, so that all of the red of the HPC report is consolidated in its dedicated module.

This Issue is to start in on this investigation, perhaps decomposing this Issue into more concrete ones as we make decisions.