catch unsupported placeholder

radicale/storage/multifilesystem/lock.py

@@ -74,8 +74,13 @@ class StoragePartLock(StorageBase):
                 else:
                     # Process group is also used to identify child processes
                     preexec_fn = os.setpgrp
+                try:
                     command = self._hook % {
                         "user": shlex.quote(user or "Anonymous")}
+                except KeyError as e:
+                    logger.error("Storage hook contains not supported placeholder %s (skip execution of: %r)" % (e, self._hook))
+                    return
+
                 logger.debug("Executing storage hook: '%s'" % command)
                 try:
                     p = subprocess.Popen(