Improve systemic errors detection by looking at the first backtrace line

What does this MR do and why?

We now look at the first line of the error backtrace to group errors, and better detect systemic errors (i.e. errors that happen systemically after a certain point due to, most probably runner environment issue, e.g. resources are exhausted, PG doesn't have enough memory etc.).

Note: There's a risk that "legit" failure messages that are a bit generic, e.g. Failure/Error: expect(job).to be_successful(timeout: 400) might be detected as systemic with this change. I'll open an MR to allow to customize the systemic error detection threshold.

MR acceptance checklist

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