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>

scripts/ectool.sh

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: GPL-3.0-only
 
 set -e
-cargo build --release --manifest-path tool/Cargo.toml
+cargo build --release --quiet --manifest-path tool/Cargo.toml
 sudo tool/target/release/system76_ectool "$@"