pkg/aflow: provide better kernel build errors

[a-nogikh]

When reporting a kernel build error, also try to include the build error report. Otherwise it's hard to understand what happened, see e.g. https://syzkaller.appspot.com/ai_job?id=55a13c60-a5f5-4f2c-8ea9-d5e1b343572c

[dvyukov]

Update #6896

[dvyukov]

I think this won't work. The problem is that we truncate errors, see #6896.
At the very least we should print buildErr first, and maybe take the last part of err.

[a-nogikh]

But the current err should actually be quite small, right?
It doesn't include the full output:

syzkaller/pkg/osutil/osutil.go

Lines 120 to 122 in 50cdcaa

	func (err *VerboseError) Error() string {
	return err.Err.Error()
	}

ExtractRootCause's output should also be reasonably small:

syzkaller/pkg/build/build.go

Lines 136 to 138 in 50cdcaa

	func (err *KernelError) Error() string {
	return string(err.Report)
	}

I think it should fit even without truncation.

[dvyukov]

We may prepend the full output later b/c what you on the dashboard is not the short output, it's full output.

[a-nogikh]

In the context of #6896, we'd probably want to keep what we keep in Error reasonably short (= do not keep full logs there) for display in the trajectory table and separately store the full log separately (and make it available via some link).

[a-nogikh]

Just in case.
@pimyn-girgis are you currently not working on sth similar in parallel?

[pimyn-girgis]

@a-nogikh I started something but I paused it for something else. You can keep going with this.