Improve summary table and GitHub messaging

[rylev]

This includes several small tweaks:

• Don't include footnotes defining terms in the triage report. Overall, it looks off, and if we decide we really need to define terms we should do it once per triage report.
• Only show errored benchmark runs in the GitHub message (it's not really relevant for the triage report).
• Show both primary and secondary benchmarks that error (we were previously only showing primary).
• Tweak the GitHub message when there are not enough changes to be significant. It is possible that there are individual changes that on their own are deemed relevant, but the comparison as a whole isn't relevant. The new messaging makes this a bit clearer by saying the "changes [are] not relevant" (i.e., point to the whole) rather than "no relevant changes".