internal/mcp/internal/util: add Wrapf

Add Wrapf, a function that helps with error wrapping.

Change-Id: I102695f75e4d217e09f828f4723cbfcbbe83f887
Reviewed-on: https://go-review.googlesource.com/c/tools/+/679177
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/util/util.go

@@ -6,6 +6,7 @@ package util
 
 import (
 	"cmp"
+	"fmt"
 	"iter"
 	"reflect"
 	"slices"
@@ -73,3 +74,10 @@ func FieldJSONInfo(f reflect.StructField) JSONInfo {
 	}
 	return info
 }
+
+// Wrapf wraps *errp with the given formatted message if *errp is not nil.
+func Wrapf(errp *error, format string, args ...any) {
+	if *errp != nil {
+		*errp = fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), *errp)
+	}
+}