Fix #977: Only treat structured exception after 2.0.

[oogFranz]

Close #977
After Elasticsearch 2.0, Elasticsearch only responds structured exception.
ref. elastic/elasticsearch#12240

The case of the issue #977 is handling structured exception as semi-structured exception which only occurs before Elasticsearch 2.0.

Ordinal code checks whether $error['error']['reason'] is set.

elasticsearch-php/src/Elasticsearch/Connections/Connection.php

Line 683 in c65c1fb

if (isset($error['error']['reason']) === true) {

But unfortunately, master_not_discovered_exception is thrown with blank error reason.
https://github.com/elastic/elasticsearch/blob/16bfdcacc0172264db9b32aa6edfa8b5c327f3b4/server/src/main/java/org/elasticsearch/discovery/MasterNotDiscoveredException.java#L31

[oogFranz]

Because too many test cases should be fixed, i close this pull-req.