KahaDB - checkForCorruptJournalFiles has hard coded 32k batch size limit in error

Using setCheckForCorruptJournalFiles=true, there is a sanity check of the journal at startup - skipping through each of the journal files validating batch record checksums and magic in an effort to detect corruption early and drop the relevant messages or error out.
There is an error in the check logic that assumes a batch record must be < 32k which is not the case if a message is > 32k

Tidy up this check and add some more detail to the error messages.