raise exception when locking the storage fails

Previously it was silently ignored, which is dangerous when multiple instances of Radicale are running.
A configuration option to disable locking was added.

radicale/config.py

@@ -139,6 +139,10 @@ INITIAL_CONFIG = OrderedDict([
             "value": "True",
             "help": "sync all changes to filesystem during requests",
             "type": bool}),
+        ("filesystem_locking", {
+            "value": "True",
+            "help": "lock the storage while accessing it",
+            "type": bool}),
         ("filesystem_close_lock_file", {
             "value": "False",
             "help": "close the lock file when no more clients are waiting",