SOLR-5905: CollapsingQParserPlugin throws a NPE if required 'field' param is missing

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1581234 13f79535-47bb-0310-9956-ffa450edef68
SOLR-5905: Changed IllegalStateException to a SolrException

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1581236 13f79535-47bb-0310-9956-ffa450edef68