Improve log message at DefaultSolrHighlighter when use.FastVectorHighlighter=true

I try use SOLR-1268 new feature. But I am confused.

I set use.FastVectorHighlighter=true, but FVH is disabled.
Because It was not set TermPosition=on and TermOffsets=on at schema.xml .

So, I suggest adding log.warn() in the following conditions.

• use.FastVectorHighlighter=true
• TermPosition=off or TermOffsets=off