tests: add test hook for SolrCmdDistributor error

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1428158 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-4290: clean up some typos, add a description (from mikes blog), null checks, and other sand

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1428159 13f79535-47bb-0310-9956-ffa450edef68