DataSender: Clarify meaning of "Update for doc ids" error

Two problems:

• The message found in Logstash didn't specify what was going on. The actual error was buried inside a sub field called 'exception' Address by stating explicitly that the "Update" failed.

• The message was not groupable making it hard to acknowledge (filter out) on a dashboard because each message was unique. Address by normalizing as '{docId}', and passing it in as before. In orde to make it easy to search in Logstash, make sure the ids are separated by a space as well as a comma. This make it compatible for Logstash purposes with the singular docId field use higher up in this class.

Change-Id: Ifbccdfee1bace5cfa4f87ef32386dcee2481ac71