improve logging for isStrictlyNewerThanDocument()

[vladak]

Lately I have been debugging a case of failed indexing due to the failed check

opengrok/opengrok-indexer/src/main/java/org/opengrok/indexer/index/IndexDatabase.java

Lines 1734 to 1741 in 4ba3aef

	//
	// Adding document with the same date of a pre-existing document which is being removed
	// will lead to index corruption (duplicate documents). Hence, make the indexer to fail hard.
	//
	throw new IndexerFault(
	String.format("attempting to add file '%s' with date matching deleted document: %s",
	path, time));
	}

and realized the isStrictlyNewerThanDocument() should really due better in terms of logging.

For the record, in my case (indexing the linux Git repository) it failed due to getDocument() returning null.