feat(helper): log error on cbcopy_helper exit

Previously, when the cbcopy_helper encountered an error, it would exit
with an error code but would not log the specific error message.
This made debugging difficult as there was no clear indication of what
went wrong.

Author: robertmu

cbcopy_helper.go

@@ -6,11 +6,13 @@ import (
 	"os"
 
 	"github.com/cloudberrydb/cbcopy/helper"
+	"github.com/greenplum-db/gp-common-go-libs/gplog"
 )
 
 func main() {
 	helper := helper.NewHelper(helper.NewConfig())
 	if err := helper.Run(); err != nil {
+		gplog.Error("cbcopy_helper exited with error: %v", err)
 		os.Exit(helper.GetErrCode())
 	}