Missleading error message when adding docs with missing/multiple value(s) for uniqueKey field

Sometimes users don't seem to notice/understand the <uniqueKey/> declaration in the example schema, and the error message they get if their documents don't include that field is confusing...

org.apache.solr.common.SolrException: Document [null] missing required field: id

...because they get an almost identical error even if they remove required=true from <field name="id" /> in their schema.xml file.

We should improve the error message so it's clear when a Document is missing the "uniqueKeyField" (not just a "required" field) so they know the terminology to look for in diagnosing the problem.

http://old.nabble.com/solr-1.4-csv-import-----Document-missing-required-field%3A-id-to26990048.html#a26990779