UnsupportedOperationException may be thrown when using custom IndexReader

If a custom IndexReaderFactory is specifiedd in solrconfig.xml, and IndexReader-s that it produces don't support IndexReader.directory() (such as is the case with ParallelReader or MultiReader) then an uncaught UnsupportedOperationException is thrown.

This call is used only to retrieve the full path of the directory for informational purpose, so it shouldn't lead to a crash. Instead we could supply other available information about the reader (e.g. from its toString() method).