psh: sysexec: "Unknown command" message is misleading

[MaciejPurski]

At this moment, when trying to execute a sysexec command with a syspage program, which is not on the white list we print a message Unknown command. It is misleading, as it suggests to the user, that the command itself does not exist or he/she made a mistake when executing the command. We should change it to for example sysexec: Command not allowed or even explicitly: sysexec: Program micropython not found on the whitelist, whichever you prefer.