Handling Trace Expansion Errors from go-corset

[DavePearce]

The prover is not properly handling error cases returned when go-corset fails in trace expansion.

Impact

I don't believe any users are affected at this time.

Describe the bug

In the file prover/zkevm/arithmetization/files.go, we have this sequence:

expTraces, errs := schema.NewTraceBuilder(sch).Build(rawTraces)
if len(errs) > 0 {
	logrus.Warnf("corset expansion gave the following errors: %v", errors.Join(errs...).Error())
}

return expTraces, nil

The problem is that, when len(errs) > 0 we have (or can have) expTraces == nil. This arises when go-corset believes it is unable to continue with trace expansion (typically because there is some column missing from the trace).

Steps to Reproduce

To reproduce this bug you need a constraint file which is missing one or more columns required by the constraints.

Actual result

The prover will panic

Expected result

The prover should fail gracefully.