Make post.jar report back detailed error in case of 400 responses

Currently post.jar does not print detailed error message that is encountered during indexing. In certain use cases, it's helpful to see the error message so that clients can take appropriate actions.

In 4.7, here's what gets shown if there is an error during indexing:

SimplePostTool: WARNING: Solr returned an error #400 Bad Request
SimplePostTool: WARNING: IOException while reading response: java.io.IOException: Server returned HTTP response code: 400 for URL: http://localhost:8983/solr/update

It would be helpful to print out the "msg" that is returned from Solr.