Initialize machine_id_checksum. Make unlocking unlocked machine a warning.

Fix issues causing errors in nightly tester:  Attempting to use
machine_id_checksum when it hasn't been initialized; and attempting to
unlock an already unlocked machine causes a fatal error.

Change-Id: Ia149953d4e25c7abf97a189bb5fdd129de865c34
Reviewed-on: https://gerrit-int.chromium.org/42489
Reviewed-by: Yunlian Jiang <yunlian@google.com>
Commit-Queue: Caroline  Tice <cmtice@google.com>
Tested-by: Caroline  Tice <cmtice@google.com>
diff --git a/lock_machine.py b/lock_machine.py
index 0f948c3..1496a2c 100755
--- a/lock_machine.py
+++ b/lock_machine.py
@@ -207,8 +207,8 @@
   def Unlock(self, exclusive, force=False):
     with FileLock(self._lock_file) as lock:
       if not lock.IsLocked():
-        self._logger.LogError("Can't unlock unlocked machine!")
-        return False
+        self._logger.LogWarning("Can't unlock unlocked machine!")
+        return True
 
       if lock.exclusive != exclusive:
         self._logger.LogError("shared locks must be unlocked with --shared")