you need to specify a field name. There is no way to search accross all fields.
In short, I don't know how to make a query to all the Document fields, because the content of each Epigraph is indexed in a dynamic field (which I know its name) but no all Documents has all of the Epigraphs (can get exceptions due to unexistent fields).
No, you won't get an exception. If the field does not exist Lucene just does not consider it for the search. If you search on a field name which does not occur in any document you just get an empty result set.