Walk: allow the visitor to return an error

[schomatis]

go-merkledag/merkledag.go

Lines 376 to 377 in bf51443

	// WalkGraph will walk the dag in order (depth first) starting at the given root.
	func Walk(ctx context.Context, getLinks GetLinks, c cid.Cid, visit func(cid.Cid) bool, options ...WalkOption) error {

Only the logic in GetLinks is allowed to but not visit.

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