SOLR-4771: Throw a more specific exception when SLF4J logging jars cannot be found on the classpath. (merged trunk r1476362)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_3@1476372 13f79535-47bb-0310-9956-ffa450edef68