cmd/vulnreport: Add the error message to the unable to label warning.

Change-Id: If6db5166175b63cee4516e0fd357a53dd6a823e6
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/600835
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

cmd/vulnreport/triage.go

@@ -120,7 +120,7 @@ func (t *triage) triage(ctx context.Context, iss *issues.Issue) {
 			log.Infof("issue #%d: would set labels: [%s]", iss.Number, strings.Join(labels, ", "))
 		} else {
 			if err := t.ic.SetLabels(ctx, iss.Number, labels); err != nil {
-				log.Warnf("issue #%d: could not auto-set label(s) %s", iss.Number, labels)
+				log.Warnf("issue #%d: could not auto-set label(s) %s\n\t%v", iss.Number, labels, err)
 			}
 		}
 		t.addStat(iss, statTriaged, "")