agent/cache: differentiate open log messages (#13362)

Changes the error output for the second open of the persistent cache
file, to differentiate it from the c.UI.Error message for the initial
open of the cache file, just to make it easier to tell where a problem
occurred.

command/agent.go

@@ -565,7 +565,7 @@ func (c *AgentCommand) Run(args []string) int {
 					AAD:     aad,
 				})
 				if err != nil {
-					c.UI.Error(fmt.Sprintf("Error opening persistent cache: %v", err))
+					c.UI.Error(fmt.Sprintf("Error opening persistent cache with wrapper: %v", err))
 					return 1
 				}