spanset: print stack with assertion error

[tbg]

It's going to be much easier figuring out what happened when the stack
is included.

See #48147.

Release note: None

[cockroach-teamcity]

This change is 

[petermattis]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @petermattis and @tbg)

---

pkg/kv/kvserver/spanset/spanset.go, line 293 at r1 (raw file):

	}

	return errors.Errorf("cannot %s undeclared span %s\ndeclared:\n%s\nstack:\n%s", access, span, s, debug.Stack())

I thought the cockroachdb/errors package grabbed stacks. Oh, this is the pkg/errors package. Hrmm.

[tbg]

@petermattis the stack is there, but you have to print with %+v. For this one I just want the stack to be part of the message for simplicity.

bors r=petermattis,nvanbenschoten

[craig]

Build succeeded

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