util: report a command name in error messages for cr_system()

It is good to know what command were executed.

https://github.com/xemul/criu/issues/371Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/util.c

@@ -652,7 +652,7 @@ int cr_system_userns(int in, int out, int err, char *cmd,
 
 		execvp(cmd, argv);
 
-		pr_perror("exec failed");
+		pr_perror("exec(%s, ...) failed", cmd);
 out_chld:
 		_exit(1);
 	}