tools/cephfs: make journal pointer/header failure explicit

iIf the journal pointer and/or header is missing or corrupt, the tool silently
returns with no-op. There is no dentry recovery happening if any of the objects
are missing but it should be explicit to the user.

This still doesn't change returning errnos on failures to maintain status quo
until https://tracker.ceph.com/issues/77913 is addressed

One important change: The condition to scan events is now changed from
header_present to header_valid. The reason for this can be understood by
reading JournalScanner::scan_header -- if the header could not be decoded, it
is set to null, using this in JournalTool::recover_dentries would segfault and
other corruptions include bad magic or inconsistent offsets. A recovery should
not proceed with distorted magic/offset.

Fixes: https://tracker.ceph.com/issues/76422
Signed-off-by: Dhairya Parmar <dparmar@redhat.com>