Change some tape server errors into warnings

Some tape server errors are not really errors. We still want a notification but the level should be demoted from ERROR to WARNING.

• ERROR: In RecallTaskInjector::reserveSpaceForNextJobBatch(): Disk space reservation failed, requeued all pending jobs. This happens when the disk is too full to recall. It's not an error, it's normal behaviour. We should keep it as a warning to notify that the disk is full, which might need to be investigated.
• ERROR: Tape thread complete for writing: this happens when we reach the end of tape. It's not an error, it's expected. But we should keep it as a warning so it can be monitored.

(What about ERROR: Tape thread complete for reading? What are the circumstances under which we get that error?)