Content Loader: Add log message if Sling-Initial-Content is not processed due to stale lock node at /var/sling/bundle-content

we had one occurrence of a very difficult to debug issue that on only one out of many instances a given bundle with Sling-Initial-Content was not processed properly when the bundle was registered by ContentLoader without any indication about the reason in the log files, even at DEBUG level.

only the message Registering bundle XYZ for content loading. was logged but nothing more, the Sling-Initial-Content was completely ignored.

we found out that the root cause was a stale "locking node" at /var/sling/bundle-content/XYZ which was probably left there after and unexpected instance shutdown or other failure. the problem was solved by deleting this locking node, but the error log did not help. we should add some more DEBUG log messages in this error to help diagnosing rare problems like this.