cmd/vulncheck_compare: missing error branch

Execution continued when `err != nil` and `binResp == nil`.
This was causing flakes on the LUCI builders

Change-Id: Ib3f9086b8315fc36b72745900720a572f2fbecce
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/612915
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Zvonimir Pavlinovic <[email protected]>

cmd/govulncheck_compare/govulncheck_compare.go

@@ -75,6 +75,7 @@ func run(w io.Writer, args []string) {
 		binResp, err := govulncheck.RunGovulncheckCmd(govulncheckPath, govulncheck.FlagBinary, binary.BinaryPath, modulePath, vulndbPath)
 		if err != nil {
 			pair.Error = err.Error()
+			continue
 		}
 		pair.BinaryResults = *binResp
 		pair.BinaryResults.Stats.BuildTime = binary.BuildTime