global sort: output clear error message (#54126)

close #54096

pkg/sessionctx/variable/sysvar.go

@@ -17,6 +17,7 @@ package variable
 import (
 	"context"
 	"encoding/json"
+	goerr "errors"
 	"fmt"
 	"math"
 	"runtime"
@@ -2537,6 +2538,11 @@ var defaultSysVars = []*SysVar{
 	}, SetGlobal: func(ctx context.Context, s *SessionVars, val string) error {
 		if len(val) > 0 && val != CloudStorageURI.Load() {
 			if err := ValidateCloudStorageURI(ctx, val); err != nil {
+				// convert annotations (second-level message) to message so clientConn.writeError
+				// will print friendly error.
+				if goerr.As(err, new(*errors.Error)) {
+					err = errors.New(err.Error())
+				}
 				return err
 			}
 		}