SOLR-4672: Requests attempting to use SolrCores which had init failures now result in 500 error responses with the details about the init failure, instead of 404 error responses (merge r1465749)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1465770 13f79535-47bb-0310-9956-ffa450edef68