tool: Add error for write locked

[crawfxrd]

Add a new error for the case of trying to flash when security is enabled and it is still locked and update the related docs.

[XV-02]

After running ectool.sh security unlock I get presented with the unlock prompt on every boot - hot or cold - until I cancel out of it.

That doesn't feel correct? I would expect it to prompt for a single boot, and then the standard locked behaviour to reassert on subsequent boots.

[XV-02]

Also, should the post ectool.sh security unlock message highlight that a reboot is insufficient to correctly change the security state?

Currently tool/src/main.rs line 299 prints "Shut down the system for the security state to take effect" which is technically correct, but could perhaps more forcibly state that a cold boot is required?