tool: Add error for write locked

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.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

tool/src/error.rs

@@ -29,6 +29,8 @@ pub enum Error {
     /// Encountered a hidapi::Error
     #[cfg(feature = "hidapi")]
     Hid(hidapi::HidError),
+    /// Writing to flash is disabled
+    WriteLocked,
 }
 
 #[cfg(feature = "std")]