nix flake lock: Fail if there is an unlocked input

Since the only purpose of `nix flake lock` is to write a new lock
file, it should be a fatal error if we can't write the lock file.

src/libflake/flake/flake.hh

@@ -156,6 +156,11 @@ struct LockFlags
      */
     bool writeLockFile = true;
 
+    /**
+     * Throw an exception when the flake has an unlocked input.
+     */
+    bool failOnUnlocked = false;
+
     /**
      * Whether to use the registries to lookup indirect flake
      * references like 'nixpkgs'.