small touches on ExtractCandidateMap

* More informative messages, so that if an error occurs, we know where
* Reading blacklistedURIs as UTF-8 for languages like Catalan
fixes for adding standard thresholds and tiny index

* getting dir from GitHub was not working. Will get a .tgz from downloads folder instead
* generating standard thresholds in index script, while we don't have an easy to run linker training
* updated path to stopwords files